Synthesis of nano-scaled yttrium aluminum garnet phosphor by co-precipitation method with HMDS treatment

Materials Chemistry and Physics 93 (2005) 79–83

Synthesis of nano-scaled yttrium aluminum garnet phosphor by co-precipitation method with HMDS treatment Yung-Tang Nien a , Yu-Lin Chen a , In-Gann Chen a,∗ , Chii-Shyang Hwang a , Yan-Kuin Su b , Shoou-Jinn Chang b , Fuh-Shyang Juang c a

Department of Materials Science and Engineering, National Cheng-Kung University, P.O. Box 26-290 Tainan, Tainan City 701, Taiwan b Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan c Department of Electro-Optics Engineering, National Huwei University of Science and Technology, Yunli, Taiwan Received 1 November 2004; received in revised form 24 January 2005; accepted 16 February 2005

Abstract The present paper describes the process of preparing nano-scaled and well-crystallized cerium-doped yttrium aluminum garnet (YAG:Ce) crystalline phosphors synthesized by co-precipitation method with the addition of hexamethyldisilazane (HMDS) as OH-scavenging reagent. Thermal analyzer and X-ray diffractometer measurements showed that pure YAG phase could be got at the temperature of 900 ◦ C lower than that by solid-state method. It was also found that smaller size crystallites (∼33 nm) were made after the HMDS treatment under the transmission electron microscope (TEM) observation. We inferred that it was achieved by reducing the surface condensing hydroxyl groups and by restricting the grain boundary motion due to second-phase particles pinning. Meanwhile, optical properties of YAG:Ce prepared in the present work were examined and showed that the photoluminescence intensity of the co-precipitated YAG:Ce crystallites plus HMDS treatment was better than those prepared by solid-state reaction or co-precipitation without any further treatment. It was proposed that the existing second-phase particles as well as higher specific surface area of nano-scaled YAG:Ce crystallites enhance the absorption of excitation. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Chemical synthesis; Luminescence

1. Introduction In recent years, much effort has been made to improve the properties of optoelectronic devices by controlling the characteristics of phosphor particles. Nano-scaled phosphor particles are required for good brightness and high efficiency. Phosphor particles prepared by conventional solid-state (SS) reaction have large size in the order of micrometer. Generally, nanocrystalline materials could be obtained by mechanical milling, such as Si [1], Ge [1,2], or ZnO [3]. However, during milling process the mechanical damage, defect and oxide species were formed to affect luminescence efficiency of the materials, which is of major importance to optoelectronic applications. Wet chemical routes were now widely used ∗

Corresponding author. Tel.: +886 6 2763741; fax: +886 6 2381695. E-mail address: [email protected] (I.-G. Chen).

0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.02.017

to form the fine phosphor particles, such as co-precipitation (CP) method [4] and sol–gel method [5,6]. However, these powders formed by polymerization of hydroxide gel species and as-dried powders by X-ray analysis are usually amorphous. A well-crystallized oxide powder will give better performance. Therefore, well-crystallized oxide powders are carried out by the heat treatment of as-dried gels at elevated temperatures and the resultant powders grow much rapidly by pulling together the constituent particles of the gel on which condensation of hydroxyl OH-groups are attached. According to Wu et al. [7], the inhibition of the crystallite growth in the sol–gel synthesis of nanocrystalline metal oxides could be achieved by introducing the OH-scavenging reagent to avoid the condensation among the surface hydroxyl groups and to restrict the grain boundaries motion due to tiny second-phase particles pinning. The rare-earth ions doped with yttrium aluminum garnet (Y3 Al5 O12 ,

80

Y.-T. Nien et al. / Materials Chemistry and Physics 93 (2005) 79–83

YAG) materials have been widely studied in the application of fluorescence displays and white light-emitting diodes because of their long lifetime and stability to electron beam and humidity [8]. In this paper, our research was focused on the preparation of nano-scaled cerium-doped yttrium aluminum garnet (YAG:Ce) crystallites via co-precipitation of metal nitrates with the addition of the OH-scavenging reagent. Meanwhile, crystalline phase, particle size and optical properties of the prepared powders were evaluated.

2. Experiment The precursor materials, Y(NO3 )3 ·6H2 O, Ce(NO3 )3 · 6H2 O (99.9%, Stream Chemicals) and Al(NO3 )3 ·9H2 O (98%, Showa Chemicals) were adopted without further purification. The process of co-precipitated (CP) yttrium aluminum garnet was referenced by Gomi et al. [9], showing that yttrium and aluminum nitrates were dissolved in 50 ml distilled water in stoichiometric ratio of 3:5 and precipitated by triethylamine [(C2 H5 )3 N, 99.9% Tedia Company Inc.], followed by filtration and drying at 90 ◦ C in the oven. The 0.3 at% of cerium relative to yttrium was added in the precursor solution for activation of yttrium aluminum garnet particles (noted as YAG:0.01Ce). The surface treatment with OH-scavenging reagent of hexamethyldisilazane [(CH3 )3 SiNHSi(CH3 )3 , 100% Ashland Chemical Inc., abbreviated as HMDS] was carried out by placing the dried gels in a closed container in which 5 ml HMDS per gram of gel was injected. The entire container was heated at 150 ◦ C for 1 h where HMDS was expected to vaporize and reacted with hydroxyl surface groups to form methyl siloxyl ones [ O Si(CH3 )3 ]. The differential thermal and thermogravimetry analysis (DTA/TG, Setaram TGA24 thermoanalyzer) were used to study the thermal decomposition and/or crystallization behaviors of the dried gels after the HMDS treatment. For firing, the dried gels containing metallic ions with first being HMDS treatment were heated at selected temperatures in the range of 700–1000 ◦ C for 1 h and subsequently quenched to determine the suitable temperature. Meanwhile, the firing time ranging from 1 to 24 h were changed to get the optimum crystallization. The infrared (IR) spectra were performed by using a Jasco FT-IR-200E spectrometer. The crystallinity of the fired powders was examined with Rigaku D/max (Cu K␣) X-ray diffractometer and identified by using powder diffraction file (No. 79-1892). The powders with and without HMDS treatment were observed by transmission electron microscopy (TEM) of Hitachi HF-2000 (200 keV) and analyzed by X-ray dispersive spectrometer equipped to scanning electron microscopy (SEM). A BET analyzer (Micromerirics ASAP2010) was used to determine the specific surface area of crystallites. For photoluminescence (PL) measurements, phosphor powders pressed in the form of pellets were excited by the wavelength of 460 nm with Shimaszu RF-5301PC spectrofluorophotometer.

3. Results and discussions The curve in Fig. 1(a) is a differential thermal analysis thermogram obtained from the precipitated gels after the HMDS treatment with a heating rate of 10 min−1 in which the broad endothermic peak below 200 ◦ C was attributed to the evaporation of water and residual triethylamine. Several exothermic peaks in the range of 200–600 ◦ C indicates the combustion of decomposed organics, and the exothermic peak near 900 ◦ C was believed to be the reaction of the YAG phase formation. Simultaneously, the thermogravimetry in Fig. 1(b) shows an overall weight loss of approximately 70% up to 600 ◦ C and reveals less change at higher temperature, which can be attributed to the removal of molecular water and decomposition of organics from the structure as revealed from the DTA curve. From DTA/TG results, it elucidates that the crystallization temperature would be reduced to lower than that by conventional solid-state process [10]. The FT-IR spectra of the co-precipitated YAG gels treated with HMDS at different temperatures are shown in Fig. 2. The band between 1200 and 1500 cm−1 for the spectra of as-precipitated and HMDS-treated gels is assigned to the stretching vibration of the C N group. The absorption intensity of this band decreases with increase in the firing temperature due to the pyrolysis of the triethylamine in the sample. The band between 3200 and 3550 cm−1 is due to the ν(O H) stretching vibration of hydroxyl groups. The absorption intensity of this band is slightly reduced in the HMDS-treated sample than that in the as-precipitated fresh sample. It was supposed that part of the surface hydroxyl groups react with vaporized HMDS and result in the formation of non-condensing methyl siloxyl ones [11]. Further experiment is needed to quantify this effect and optimize the reaction. The peaks at about 797 and 690 cm−1 represent the characteristics of ν(Al O) metal–oxygen vibrations, while the peaks at about 718, 569 and 458 cm−1 represent the characteristics of ν(Y O) metal–oxygen vibrations [12–14]. These characteristic peaks appear at around 900 ◦ C and become much sharper with the increasing firing temperature.

Fig. 1. DTA(a) and TG(b) analysis of as-precipitated metal gels after HMDS treatment. Exothermic peak at about 925 ◦ C in curve (a) indicated the formation of yttrium aluminum garnet phase (YAG).

Y.-T. Nien et al. / Materials Chemistry and Physics 93 (2005) 79–83

81

Fig. 2. FT-IR spectra of the as-precipitated YAG samples and treated with HMDS at different firing temperatures.

These results indicating the decomposition of starting organics and the appearance of YAG fingerprints are in agreement with the thermal analysis as previous discussion. Fig. 3(a) shows the X-ray diffraction patterns of HMDStreated samples fired at different temperatures for 1 h in the range of 700–1000 ◦ C. It was found that no diffraction peaks appear at 800 ◦ C, indicating that below this temperature the sample is amorphous. The diffraction pattern of the sample fired at 900 ◦ C shows that all the peaks are corresponding to the cubic garnet structure of YAG phase (JCPDS no. 791892), and no other crystalline phase can be detected, such as YAM (monoclinic) or YAP (perovskite). It reveals that the intensity of the YAG diffraction peaks are enhanced due to the improvement of crystallinity for further heating at higher temperature as well as for longer firing time as shown in the inset of Fig. 3(b). No differences for the formation of the crystalline YAG phase can be observed in the sample doped with Ce3+ ion, indicating that some sites of the Y3+ ions could be replaced by the Ce3+ ions based on their identical valence and similar radius. In conventional solid-state reaction between the component oxides, a high firing temperature (higher than 1600 ◦ C) or prolonged firing time is required to gain the pure YAG phase. However, the chemical process as discussed here achieves intimate mixing of reactant cations at the atomic level, leading to an increase of reaction rate and a decrease of synthesized temperature. The TEM images of YAG crystallites synthesized by firing of dried metal gels with HMDS treatment or not are shown in Fig. 4(a) and (b), respectively. Fig. 4(a) reveals that the YAG crystallites with HMDS treatment are of round shape and smaller than 33 nm. The inset of Fig. 4(a) shows the corresponding nano area electron diffraction pattern from these crystallites reflecting the presence of single cubic phase. In contrast, the YAG crystallites without HMDS treatment shown in Fig. 4(b) tend to grow much larger than 50 nm

Fig. 3. X-ray diffraction patterns of YAG samples fired at different temperatures (a) and firing time (b). The inset in (b) shows the (4 2 0) peak intensity, i.e. crystallinity, as a function of firing time.

and to sinter as polycrystalline aggregations indicated by the neck growth. The specific surface area of the prepared YAG crystallites without and with HMDS treatment measured by BET analyzer was 23.6 and 37.3 m2 g−1 , respectively. The increased specific surface area was considered to be due to the decreased size of YAG crystallites after HMDS treatment. By examining with an energy-dispersive spectrometer (EDS) equipped to scanning electron microscope, silicon composition was found in the HMDS-treated YAG crystallites. Therefore, the silica SiO2 which was formed from methyl siloxyl groups [ O Si(CH3 )3 ] at elevated temperature and was regarded as mostly amorphous phase, was considered to play an important role in suppressing diffusion between YAG crystallites and prohibiting grain growth during firing process. A similar phenomenon was reported for oxides containing SiO2 fabricated by thermal hydrolysis under hydrothermal condition [15]. The photoluminescence of YAG:0.01 Ce crystallites synthesized at 980 ◦ C by co-precipitated process with HMDS treatment or not was carried out at room temperature. The

82

Y.-T. Nien et al. / Materials Chemistry and Physics 93 (2005) 79–83

Fig. 4. TEM images of YAG crystallites synthesized at 980 ◦ C by co-precipitated process: (a) HMDS treatment; (b) no HMDS treatment. The inset of (a) shows the nano area diffraction pattern for the powder [scale bars are 33 and 50 nm in (a) and (b), respectively].

broad green-yellow emission band peaked at 527 nm shown in Fig. 5 is ascribed to the electron transitions from the excited state of 2 Dj to the ground states of 2 F5/2,7/2 of Ce3+ ions in the Y3 Al5 O12 host material [16]. From Fig. 5, it can be seen that there is no difference in the shape of emission spectra for samples with and without HMDS treatment. However, it is revealed that the emission intensity of HMDS-treated crystallites is higher than that of ones without HMDS treatment. This phenomenon was believed to be partly resulted from the quantum confinement effect due to smaller size and higher specific surface area of nano-scaled crystallites after HMDS treatment as indicated by TEM micrographs (Fig. 4) and BET analysis. The emission spectrum noted as SS in Fig. 5 was the YAG:0.01 Ce sample with the size of 2–4 ␮m synthesized by solid-state reaction of Y2 O3 , Al2 O3 and CeO2 at elevated temperature in air. In Fig. 5, the resultant photoluminescence

Fig. 6. Diffuse reflectance spectra of YAG:0.01 Ce prepared by solid-state (SS) reaction and co-precipitated (CP) process plus HMDS treatment, respectively.

intensity was much lower than those by CP process. The diffuse reflectance measurement in Fig. 6 shows that YAG:0.01 Ce crystallites by CP plus HMDS treatment was performed with less reflectivity in the whole spectrum than those by SS method. It is supposed that SiO2 formed in the YAG powders from HMDS would not only suppress the grain growth, but also appeared as in Fig. 6 enhanced the absorption of excitation wavelength of 460 nm and resulted in higher emission intensity. Meanwhile, the intense absorption in the UV band was found in the samples by CP plus HMDS treatment. The absorption at 340 nm was attributed to the YAG host material; the abrupt absorption in the shorter wavelength near 320 nm was due to the inter-valence charge-transfer transition between Ce3+ and Ce4+ [17,18], which was resulted from higher chemical activity of nano-scaled YAG:0.01 Ce crystallites.

4. Conclusions

Fig. 5. Photoluminescence spectra of YAG:0.01 Ce prepared by solid-state (SS) reaction and co-precipitated (CP) process plus HMDS treatment, respectively.

The nano-scaled YAG:Ce crystallites were synthesized by co-precipitation method with HMDS treatment at lower firing temperature of 900 ◦ C. The FT-IR results showing less absorption between 3200 and 3550 cm−1 confirmed that HMDS

Y.-T. Nien et al. / Materials Chemistry and Physics 93 (2005) 79–83

reacted with surface hydroxyl groups of precipitated gels. It was also analyzed to confirm the appearance of single cubic YAG phase by XRD and to estimate the size of 33 nm by TEM observation. The photoluminescence measurements excited by the wavelength of 460 nm show that the emission intensity of the HMDS-treated YAG:Ce crystallites was higher than that of ones without HMDS treatment. It is supposed that the SiO2 resulted from replacing of surface hydroxyl group with non-condensing methyl siloxyl group not only obstructed the crystallite growth by trapping grain boundaries, but also enhanced the absorbance of excitation as well as the diffused reflection of emission. Therefore, higher luminescence intensity of YAG:Ce phosphors was achieved by the HMDS treatment of dried precipitated gels before firing process.

Acknowledgements This work was supported in part by both the Center for Micro/Nano Technology Research, National Cheng Kung University, under projects from the Ministry of Education and the National Science Council (NSC 93-212-M-006-006) and (NSC-92-2215-E006-004) of Taiwan.

83

References [1] R. Radoi, P. Fern´andez, J. Piqueras, M.S. Wiggins, J. Sol´ıs, Nanotechnology 14 (2003) 794. [2] Nae-Lih Wu, Sze-Yen Wang, I.A. Rusakova, Science 285 (1999) 1375. [3] C. Diaz-Guerra, A. Montone, J. Pigueras, F. Cardellini, Semicond. Sci. Technol. 17 (2002) 77. [4] E. Nogales, A. Montone, F. Cardellini, B. M´endez, J. Piqueras, Semicond. Sci. Technol. 17 (2002) 1267. [5] T. Igarashi, T. Isobe, M. Senna, Phys. Rev. B 56 (1997) 6444. [6] R.P. Rao, J. Electrochem. Soc. 143 (1996) 189. [7] S. Erdei, R. Roy, G. Harshe, H. Juwhari, D. Agarwal, F.W. Alinger, W.B. White, Mater. Res. Bull. 30 (1995) 745. [8] K. Ohno, T. Abe, J. Electrochem. Soc. 133 (1986) 638. [9] M. Gomi, T. Kanie, Jpn. J. Appl. Phys. 35 (1996) 1798. [10] J.S. Abell, J. Mater. Soc. 9 (1974) 527. [11] C.M. Jin, J.D. Luttmer, D.M. Smith, T.A. Ramos, Mater. Res. Bull. 22 (1997) 39. [12] P. Apte, H. Burke, H. Pickup, J. Mater. Res. 7 (1992) 706. [13] V. Saraswati, G.V.N. Rao, G.V. Rama Rao, J. Mater. Sci. 22 (1987) 2529. [14] P. Colomban, J. Mater. Sci. 24 (1989) 3002. [15] M. Hirano, K. Ota, H. Iwata, Chem. Mater. 16 (2004) 3725. [16] J. Lin, Q. Su, J. Mater. Chem. 5 (1995) 1151. [17] G. Blasse, Struct. Bond. 76 (1991) 153. [18] J. Lin, G. Yao, Y. Dong, B. Park, M. Su, J. Alloys Compd. 225 (1995) 124.