Preparation and luminescence of Sr[AL.sub.2][O.sub.4]: [Eu.sup.2+] [Dy.sup.3+] phosphors coated with maleic anhydride.

Shengfei, Yu, ; Pihui, Pi ; Xiufang, Wen 等

INTRODUCTION

Sr[Al.sub.2][O.sub.4]: [Eu.sup.2+], [Dy.sup.3+] (SAO-ED) phosphor is one of the photoluminescence (PL) inorganic compounds material. SAO-ED has been widely used in luminous plastic, printing ink, paints, and ceramic industries as previously shown (Luo et al., 2002) due to its high quanta efficiency, long afterglow life, better safety, no radiation, and good chemical stability compared with the traditional sulphide phosphors as previously shown (Chang et al., 2003). However, it should be noted that the SAO-ED phosphor is liable to hydrolyze in water or wet air, which resulted in decrease in luminous intensity and shortening of afterglow life (Yang et al., 2004). Therefore, SAO-ED must be coated with a protective layer when it is used in a water-containing environment.

Organic coating technology on the surface of the phosphor is a branch of power coating technology. The coating layer can improve the phosphors' water resistance and its dispersing ability in organics (Schottland et al., 2004). The organic coating technology is mainly classified as direct coating with macromolecule polymer film (Chan et al., 1998), polymerization film with organic monomer as previously shown (Bryan et al., 1997), and adsorption coating with organic compounds as previously shown (Akiteru, 1997). The interaction force between the coating and SAO-ED phosphors is van der Waals force or electrostatic force, which is very weak and the coating layer can easily separate from the substrate. In 2005, X. D. Lu and W G. Shu successfully achieved chemical coating through the link of silane coupling agent as previously shown (Lu and Shu, 2005). In Lu and Shu's experiment, the SAO-ED phosphors is pre-treated with 3-(triethoxysilyl) propyl methacrylate, and then SAO-ED is coated with polymer film by polymerization of methyl methacrylate and acrylic acid on the phosphors. It has been found that the coating layer effectively improved the phosphors' water resistance. In general, this two-step method use the Si-O-Al bond to bond the coating layer and SAO-ED phosphors.

In this paper, the coordination chemistry method, which has been used to prepare the inorganic-organic hybrid materials as previously shown (Chen, 2001), is introduced and first used in surface treatment of SAO-ED phosphors. The O atom of maleic anhydride is in coordination with metal ions on the surface of the phosphors. The coating layer can effectively improve phosphors' water resistance at the cost of a minor loss of initial brightness and lengthen the afterglow life. In fact, this process is only accomplished in one step, therefore the process can be more easily controlled.

EXPERIMENTAL

In this study, SAO-ED phosphor (Model PLO-6C, grain size 20-30 [micro]m) is used as coated material and it is obtained from Dalian Luminglight Science Technology Co. Ltd. (Dalian, China). Maleic anhydride, chloroform, and surfactant ethoxy alkyl sulphosuccinate (A-102) (all are analytical grade) was obtained from Guangzhou Xingang Chemical Co. Ltd. (Guangzhou, China), Tianjin Yuanli Chemical Co. Ltd., and Guangzhou Shuangjian Trade Co. Ltd. (Guangzhou, China), respectively.

During the coating process, 5.0 g SAO-ED and 30 wt% maleic anhydride is first added into 50.0 ml, chloroform and is ultrasonically dispersed for 15 min, then the suspension is put into a three-necked flask and is heated at a rate of 1.2[degrees]C/min. In the mean time, 1 wt % A102 is added slowly through a dropping funnel. When the suspension temperature reached 50 [+ or -] 1.0[degrees]C, the suspension was continuously stirred steadily for 30 min at the constant temperature of 50[degrees]C. After that, the pH of the suspension is adjusted to 9.5 by using ammonia with a concentration 25 wt %. Such generated suspension was stirred for another 9 h before the coordination coating process can be completed. This suspension is first sieved with a filter press and washed several times with chloroform, then dried for 4 h at 80[degrees]C. SAO-ED (5.5 g) phosphors coated with maleic anhydride are then yielded.

Infrared absorption spectrum of powders is recorded by using a Fourier transition spectrometer (Model Vector33) with a KBr-pressed disk. Surface element of the phosphors was analyzed by X-ray photoelectron spectra (XPS, Model Axis Ultra DLD). The crystal structure of powders is characterized by X-ray diffraction (XRD, D/max-IIIA). Powder microstructure of phosphors was observed via a scanning electron microscopy (SEM, Model FEI XL-30). The PL properties of phosphors are measured via a fluorescent spectrometer (Model F-4500). Prior to afterglow decay measurement with a surface brightness meter (Model ST -861,A), the samples are pre-excited by a fluorescent lamp with the power of 40 W for 10 min, and the distance between the phosphors and the lamp is about 20 cm. The pH value was measured with a digital acidity meter (Model PHS-25) to evaluate the stability of the phosphors in water.

[FIGURE 1 OMITTED]

RESULTS AND DISCUSSION

Fourier Transform Infrared Spectroscopy (FTIR)

Maleic anhydride containing O atoms, which can provide electrons, can coordinate with unsaturated metal ions ([Sr.sup.2+], [Al.sup.3+], [Eu.sup.2+], [Dy.sup.3+]) on the surface of SAO-ED phosphors. The absorbance of hydroxyl group -OH at 3450 [cm.sup.-1] of coated phosphors (Figure 1b) is broader than that of uncoated phosphors (Figure la) because of the presence of carboxyl group of coated phosphors. The absorbance of carbonyl group C=O shifted from original wave number 1850, 1790 [cm.sup.-1] (Figure 1c) to 1554 (vasCO[O.sup.-]), 1416 [cm.sup.-1] (vsCO[O.sup.-]) (Figure 1b) due to the strong interaction of the coordination bond as previously shown (Liu et al., 2002), and the absorbance of C-O also shifted from original wave number 12801, 1230 [cm.sup.-1] to 1250, 1200 [cm.sup.-1] due to the vibration of the coordination group. Moreover, the gap between two C=O vibration peaks of coated phosphors 140 [cm.sup.-1] is larger than that of uncoated phosphors 60 [cm.sup.-1], which indicates the carboxyl groups of malay acid are coordinated with the metal ions on the surface of SAO-ED phosphors in a monodentate fashion as previously shown (Nakamoto, 1991). In addition, the absorbance peaks at 1650 [cm.sup.-1] due to the vibration of ethylene -C=C-, yet does not peak at 2930, 2850 [cm.sup.-1] due to the vibration of methyl and methylene -CH2, which showed polymerization of alkene alkyl did not happen. This demonstrated that maleic anhydride coated on phosphors, and the coating process is an interfacial coordination chemistry process. Metal ions are coordinated with hydroxyl groups O atom of and carboxyl groups O atom of hydrolysis product of maleic anhydride to coordinate saturation, and the polymer coating layer is caused by O atom of hydroxyl group of malay acid bridging with carboxyl group as previously shown (Asato et al., 1993).

XPS

The presence of coordinated ligands on the surface of SAO-ED particles can be further proved by signals arising from the carbon and oxygen and the metal ions [Al.sup.3+] and [Sr.sup.2+]. The binding energy of A12p increases from 73.94 (Figure 2b) to 74.05 eV (Figure 2c). The binding energy of Sr3d5/2 and the Sr3d3/2 increase from 132.99 and 134.72 (Figure 2b) to 133.23 and 134.98 eV (Figure 2c), respectively. In the mean time, the binding energy of carbonyl group C1s reduced from 289.26 to 288.35 eV and the alkene alkyl C1s kept almost the same. The reason for the above-mentioned phenomena could be that the maleic anhydride coordinates with the metal ions [Al.sup.3+] and [Sr.sup.2+] to form M-O-C (M is [Al.sup.3+] and [Sr.sup.2+]) coordination bond, in which the outer electron cloud of M shifts to O and to C finally as previously shown (Wu et al., 2007). The binding energy of O1s is difficult to differentiate, but the binding energy of carboxyl group O of malay acid coordinated with Al at 532.35 eV and hydroxyl group O of malay acid coordinated with Al at 530.65 eV is found as previously shown (Leung et al., 1992). All these indicate that the coordination forms of metal ions on the surface of SAO-ED phosphors with malay acid are M-O [right arrow] C and M [left arrow] O. During the coating process, malay acid is adsorbed chemically on the surface of the phosphors by the coordination bond, the following malay acid further is adsorbed by bridging O atom so that a layer of continuous and dense maleic anhydride film came into being.

[FIGURE 2 OMITTED]

SEM

SEM micrographs of uncoated and coated phosphors are shown in Figure 3. It can be seen that the surface of uncoated SAOED phosphors is smooth and clean (the white dot could be adsorbed smaller phosphor particles since the surface effect of super fine particles or impurity), which is shown in Figure 3a. After being treated with maleic anhydride, the surface of the phosphors becomes rougher and is covered with dense maleic anhydride film (shown in Figure 3b).

Water Resistance

The phosphors are dispersed in deionized water (the ratio of the phosphors to deionized water was 10 wt%) under mild stirring at constant ambient temperature for 30 d. The pH value of the solution is measured daily to evaluate the water resistance of SAOED phosphors. This is because aluminate substrate in phosphors carries out the reaction as following in water:

Sr[Al.sub.2][O.sub.4] + [H.sub.2]O [right arrow] [Sr.sup.2+] + 2O[H.sup.-] + [Al.sub.2][O.sup.3] [down arrow] (1)

The resultants include the deposition aluminum hydroxide and soluble strontium hydroxide. The [OH.sup.-] concentration denotes the degree of hydrolysis of SAO-ED phosphors. This also indicates the pH value in water solution increase with the degree of hydrolysis of SAO-ED phosphors.

[FIGURE 3 OMITTED]

Figure 4 shows the relationship between pH value in water solution and dipping time. It was found that uncoated phosphors hydrolyze completely within several minutes and the pH value rises rapidly. However, the pH value reached equilibrium at 13.5. After coated with maleic anhydride, water resistance of phosphor has been significantly improved. In the mean time, the coated phosphors only hydrolyze slightly within 1 h, after that the pH value reach equilibrium at 7.0.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

XRD

Figure 5a shows the XRD curves of the coated and uncoated SAOED phosphors. It can be seen that the coated phosphor has no obvious peaks of maleic anhydride, and only some negligible peaks of maleic anhydride are found, which implies that maleic anhydride coating layer is very thin or maleic anhydride on the surface of phosphor are amorphous. As mentioned in the previous section, the coating layer is very dense, so maleic anhydride coated on the SAO-ED phosphors are amorphous. The formation of the coating layer do not change the crystal structure. Figure 5b shows the XRD curves of the coated and uncoated SAO-ED phosphors after they are dipped into water for one month. The XRD curve of dipped coated phosphors is the same as that of non-dipped coated phosphors, which indicates that the crystal structure of coated phosphors is not changed after it is dipped into water for one month. This is because that the coating layer isolates the phosphors from water. However, the XRD curve of dipped uncoated phosphors is different from that of non-dipped uncoated phosphors. In the new XRD curve, there appears the structure of [Sr.sub.3][Al.sub.2](OH) 12 and Sr[Al.sub.3][O.sub.5](OH). [Sr.sub.3][Al.sub.2](OH) 12 is completely soluble in water and does not emit light. Sr[Al.sub.3][O.sub.5](OH) is insoluble in water. This result suggests that uncoated phosphors can decompose into the mixture of [Sr.sub.3][Al.sub.2](OH)12 and Sr[Al.sub.3][O.sub.5](OH) after it is dipped into water for one month and its crystal structure is destroyed, which is supported by the previous experimental study (Yang et al., 2004).

[FIGURE 6 OMITTED]

Photoluminescence

Figure 6a shows the results of emission and excitation spectra of uncoated and coated phosphors. It is seen that both samples have one emission peak centred at 512 nm under the excitation of 364 nm wavelength, as a result of the [4f.sup.7] (SS) to [4f.sup.6][5d.sup.1] transition of [Eu.sup.2+] ions in Sr[Al.sub.2][O.sub.4], but peak intensity of coated phosphors is a little weaker than that of uncoated phosphors. The reason may be that, for the phosphor coated with maleic anhydride, light emitted from light source is absorbed partially by the maleic anhydride and light intensity through maleic anhydride coating layer decreases. That is to say, light intensity emitted to phosphor nuclei is decreased, which induces that the excitation intensity of the phosphors decreases and the emission intensity of the phosphors decreases. Moreover, light emitted from the phosphors nuclei is little lost when it penetrates through maleic anhydride coating layer, which leads to the reduction of PL intensities of phosphor. Figure 6b shows the results of emission and excitation spectra of the coated and uncoated SAO-ED phosphors after they were dipped into water for one month. The position and shape of emission and excitation peak of dipped coated phosphors is the same as that of non-dipped coated phosphors, which indicates that the crystal structure of coated phosphors has not been changed after it is dipped into water for one month. However, the dipped uncoated phosphors have one emission peak centred at 497 nm under the excitation of 347 nm wavelength, which is different from that of non-dipped uncoated phosphors. These conclusions are in agreement with the results of above XRD analysis.

[FIGURE 7 OMITTED]

From afterglow decay curves of emission peak of the coated and uncoated phosphors (Figure 7a), it can be found that both of them have similar decay speed and persistent life, but initial brightness of coated phosphors is slightly weaker than that of uncoated phosphors. However, after they are dipped into water for one month, the phenomenon is reverse. The initial brightness of coated phosphors is much stronger than that of uncoated phosphors, and the decay speed of coated phosphors is much slower than that of uncoated phosphors (Figure 7b).

CONCLUSIONS

The interfacial coordination chemistry method for maleic anhydride coating on the surface of SAO-ED phosphors is feasible for the surface modification of the phosphors. FTIR and XPS and SEM results showed that metal ions of SAO-ED phosphors are coordinated with carboxyl group O atom of hydrolysis product of maleic anhydride and with hydroxyl group O atom, and dense polymer maleic anhydride coated on the surface of SAO-ED phosphors by the bridging O atom. XRD analysis showed that the maleic anhydride-coating layer does not change the crystal structure of SAO-ED phosphors, and it can also protect the SAOED phosphors from water, which improve water resistance of the phosphors. The excitation and emission spectra and afterglow decay of the phosphors does not change when the maleic anhydride is introduced. After the coated phosphors are dipped into water, the PL and water resistance are improved.

Manuscript received April 2, 2007; revised Manuscript received June 11, 2007; accepted for publication July 27, 2007.

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Shengfei Yu, (1,2), Pihui Pi (1), Xiufang Wen (1), Jiang Cheng (1) and Zhuoru Yang (1)

1. School of Chemical and Energy Engineering, South China University of Technology, Guangzhou 510640, China

2. School of Chemical and Environment Engineering, Shaoguan University, Shaoguan 512005, China

* Author to whom correspondence may be addressed. E-mail address: yusherigfei@a torn.com

DOI 10.1002/cjce.20012