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Fabrication of silica nanocoatings on ZnS-type phosphors via a sol–gel route using cetyltrimethylammonium chloride dispersant
Materials Letters 60 (2006) 1284 – 1286 www.elsevier.com/locate/matlet
Fabrication of silica nanocoatings on ZnS-type phosphors via a sol–gel route using cetyltrimethylammonium chloride dispersant Jiongliang Yuan a,⁎, Koji Kajiyoshi a , Kazumichi Yanagisawa a , Hideki Sasaoka b , Kazuhito Nishimura b a
Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, 2-5-1, Akebono-cho, Kochi 780-8520, Japan b Kochi Industrial Promotion Center, 3992-2, Nunoshida, Kochi 781-5101, Japan Received 12 September 2005; accepted 6 November 2005 Available online 28 November 2005
Abstract In order to apply ZnS-type phosphors in field emission displays (FEDs), their poor ageing performance, resulting from their surface oxidation at high current densities, should be improved. In this study, the green emitting ZnS:Ag,Cl phosphors are covered with uniform and continuous SiO2 coatings via a sol–gel route, which is expected to inhibit their surface oxidation. During the gelation process, cetyltrimethylammonium chloride (CTAC), a cationic surfactant, is added to increase the dispersibility of phosphors in suspension. Furthermore, the addition of CTAC promotes uniform distribution of charges on the whole phosphor surfaces, thus benefit the formation of continuous and uniform coatings. © 2005 Elsevier B.V. All rights reserved. Keywords: ZnS phosphors; Degradation; Silica; Cetyltrimethylammonium chloride; Nanocoating; Sol–gel
1. Introduction Recently, field emission displays (FEDs) have been developed as next generation flat panel displays to replace crystal ray tubes (CRTs). But worldwide attempts to develop new, durable, and high-efficiency phosphors have not yet been successful so far. ZnS-type phosphors have been widely used in crystal ray tubes (CRTs) because of their higher cathodoluminescent (CL) properties. However, they encounter the problem of fast degradation at higher current densities in the FED environment because FEDs operate at much lower voltage excitation, which requires higher current densities in order to maintain the same output luminance. The poor ageing performance is believed to be associated with the surface recombination of phosphors [1,2]. According to electron stimulated surface chemical reaction (ESSR) mechanism, the degradation is caused by the surface recombination of
⁎ Corresponding author. Present address: Department of Environmental Engineering, Beijing University of Chemical Technology, Beisanhuan Dong Lu 15, Beijing 100029, China. Tel.: +86 10 63651193; fax: +86 10 64713931. E-mail address: [email protected] (J. Yuan). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.11.015
phosphors (reaction with residue gases in the vacuum). Because of the existence of traces of oxygen, ZnS surfaces will be oxidized into ZnO dead layers which reduce cathodeluminescence (CL) intensity significantly. One way to slow the degradation rate is to coat phosphor surfaces with protective layers which can isolate the surfaces from the residue gases in the FED environment. Such coatings as In2O3 [3], MgO [4], TaSi2 [5], BaTiO3 [6] and calcium polyphosphate [7] have been investigated. However, none of those coatings is applicable. For example, In2O3 coatings can improve CL performance only at very low voltage (b 400 V) [3], the anti-degradation effect of calcium polyphosphate coatings is pronounced at midvoltage range (approximately 4 kV) [7], and aluminum compound coatings will cause a small change in color chromaticity [4]. Silica is expected to be one of the candidates for coatings because it has energy band gap larger than ZnS, and the wide band gap layer can reflect the electrons generated by the incident electron beam from the defective high surface recombination velocity region back into the phosphors [8]. In this paper, silica coatings on phosphors have been obtained via a sol–gel route using cetyltrimethylammonium chloride (CTAC) dispersant.
J. Yuan et al. / Materials Letters 60 (2006) 1284–1286
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2. Experimental ZnS-type phosphors used in this work are green emitting ZnS:Ag,Cl phosphors which mean particle size is 6.5 μm (Nichia Co., Japan). The silica coatings on phosphors are fabricated by sol–gel method using tetraethyl orthosilicate (TEOS) as the precursor. In a typical experiment, TEOS is first dissolved in the mixture of ethanol and distilled water, and pH value of the mixture is adjusted by diluted nitric acid. The solation process is conducted at 50–75 °C for 10 min. Together with the additive CTAC, a certain amount of green phosphors are suspended in distilled water and deagglomerated by treating for 5 min with ultrasonics. In order to get silica coatings on phosphors, the hydrolysis product is added to the suspension, and stirred for hours. The resulting product is separated from the solution, washed twice with ethanol and dried at 100–110 °C for 1 h, thereafter annealed at 400 °C in air for 1 h. Zeta potential of raw phosphors without and with the addition of CTAC is measured using Model 502 Zeta Potential Instrument (Rufuto Co. Ltd., Japan). Samples are dispersed into distilled and ion exchanged water, then pH of sample suspension is adjusted by adding very dilute hydrochloride solution or sodium hydroxide solution. In order to deagglomerate, sample suspension is treated by ultrasonication for 15 min. A JSM-5800 scanning electron microscopy (JEOL, Japan) and a HITACHI H-800 transmission electron microscopy (Hitachi Co., Japan) are used for microstructure observation, respectively. 3. Results and discussions Zeta potential of raw phosphors without and with the addition of CTAC as the function of pH is measured as shown in Fig. 1. This shows that the isoelectric point (IEP) of raw phosphors without the addition of CTAC is at pH ca. 3.1, which is different from that of ZnS phosphors in other literatures (pH ca. 7.0) [9,10]. It might be because the as-received commercial ZnS phosphors were already treated with particulate-like silica to improve their fluidity when making screens [11]. In our experiments, the coating process is conducted at pH 2–4, which is very near to IEP of raw phosphors. At the adjacency of IEP, phosphor powders are easy to agglomerate together because the
Fig. 1. Zeta potential of raw phosphors without and with the addition of CTAC as the function of pH.
Fig. 2. SEM images of uncoated (a) and SiO2 coated (b) phosphors.
repulsive force among phosphors is very weak. In order to increase the dispersibility of phosphors, CTAC, a cationic surfactant, is added into phosphor suspension. Because the special adsorption of CTAC molecules on the phosphors, the absolute value of zeta potential increases (Fig. 1), that is to say, the repulsive force among phosphors increase significantly at that dosage of CTAC. With the addition of CTAC, phosphor powders are very easy to disperse, which will promote the coating process. The scanning electron microscopy (SEM) images of uncoated and SiO2 coated phosphors are presented in Fig. 2. As shown in Fig. 2(a), there are many small spots on raw materials, which diameter is estimated to be about 200 nm. Compared with raw phosphors, the surfaces of coated phosphors look smoother because small spots on raw phosphors are partially covered with SiO2 coatings (Fig. 2(b)). The transmission electron microscopy (TEM) images of uncoated and SiO2 coated materials are presented in Fig. 3. Fig. 3 shows that there is a transparent uniform and continuous film on the coated phosphor compared with the raw phosphor. That coating is expected to inhibit the surface oxidation of ZnS. Merikhi and Feldmann reported that colloidal SiO2 particles adhere on ZnS-type phosphor surfaces [10]. Unfortunately, rather than uniform and continuous coatings, the phosphors are covered by island-like SiO2 particles, which cannot be expected to improve the ageing performance significantly because of uncompleted coverage. In literature [10], colloidal ZnO particles are first attached to phosphor surfaces, and colloidal SiO2 particles to ZnO particle surfaces by electrostatic attractive force, in which ZnO particles act as bridges between ZnS phosphors and colloidal SiO2 particles. Because phosphor surfaces are partially covered by ZnO particles, it is difficult
Fig. 3. TEM images of uncoated (a) and SiO2 coated (b) phosphors.
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to get complete coverage of SiO2. But in this study, raw phosphor surfaces are uniformly covered with CTAC molecules, thus the electric properties of the surfaces become more uniform, which will benefit the uniform gelation process of silicate acid sol on the whole surfaces. Furthermore, a uniform and continuous coating ascribes uniform particle surfaces and appropriate homogeneous nucleation rate. The gelation rate is slow enough to get a continuous film because of the precisely adjusted concentration of precursor solution in this study.
4. Conclusions The green emitting ZnS:Ag,Cl phosphors are covered with uniform and continuous SiO2 coatings by sol–gel method. The special adsorption of CTAC on raw phosphors increases the absolute value of zeta potential, which is helpful, not only for the dispersibility of phosphors but also for the formation of uniform and continuous coatings. Now we are trying to get thinner coatings with better ageing performance.
References [1] S. Itoh, T. Kimizuka, T. Tonegawa, J. Electrochem. Soc. 136 (1989) 1819. [2] K.T. Hillie, S.S. Basson, H.C. Swart, Appl. Surf. Sci. 187 (2002) 137. [3] H. Kominami, T. Nakamura, Y. Nakanishi, Y. Hatanaka, Jpn. J. Appl. Phys. 35 (1996) L1600. [4] R.Y. Lee, S.W. Kim, J. Lumin. 93 (2001) 93. [5] J.M. Fitz-Gerald, T.A. Trottier, R.K. Singh, P.H. Holloway, Appl. Phys. Lett. 72 (1998) 1838. [6] T. Nakamura, M. Kamiya, H. Watanabe, J. Electrochem. Soc. 142 (1995) 949. [7] H. Bechtel, W. Czarnojan, M. Haase, W. Mayr, H. Nikol, Philips J. Res. 50 (1996) 433. [8] W. Park, B.K. Wagner, G. Russell, K. Yasuda, C.J. Summers, Y.R. Do, H. G. Yang, J. Mater. Res. 11 (2000) 2288. [9] C. Feldmann, J. Merikhi, J. Colloid Interface Sci. 223 (2000) 229. [10] J. Merikhi, C. Feldmann, J. Colloid Interface Sci. 228 (2000) 121. [11] J.-C. Souriau, Y.D. Jiang, J. Penczek, H.G. Paris, C.J. Summers, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 76 (2000) 165.
Fabrication of silica nanocoatings on ZnS-type phosphors via a sol–gel route using cetyltrimethylammonium chloride dispersant Jiongliang Yuan a,⁎, Koji Kajiyoshi a , Kazumichi Yanagisawa a , Hideki Sasaoka b , Kazuhito Nishimura b a
Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, 2-5-1, Akebono-cho, Kochi 780-8520, Japan b Kochi Industrial Promotion Center, 3992-2, Nunoshida, Kochi 781-5101, Japan Received 12 September 2005; accepted 6 November 2005 Available online 28 November 2005
Abstract In order to apply ZnS-type phosphors in field emission displays (FEDs), their poor ageing performance, resulting from their surface oxidation at high current densities, should be improved. In this study, the green emitting ZnS:Ag,Cl phosphors are covered with uniform and continuous SiO2 coatings via a sol–gel route, which is expected to inhibit their surface oxidation. During the gelation process, cetyltrimethylammonium chloride (CTAC), a cationic surfactant, is added to increase the dispersibility of phosphors in suspension. Furthermore, the addition of CTAC promotes uniform distribution of charges on the whole phosphor surfaces, thus benefit the formation of continuous and uniform coatings. © 2005 Elsevier B.V. All rights reserved. Keywords: ZnS phosphors; Degradation; Silica; Cetyltrimethylammonium chloride; Nanocoating; Sol–gel
1. Introduction Recently, field emission displays (FEDs) have been developed as next generation flat panel displays to replace crystal ray tubes (CRTs). But worldwide attempts to develop new, durable, and high-efficiency phosphors have not yet been successful so far. ZnS-type phosphors have been widely used in crystal ray tubes (CRTs) because of their higher cathodoluminescent (CL) properties. However, they encounter the problem of fast degradation at higher current densities in the FED environment because FEDs operate at much lower voltage excitation, which requires higher current densities in order to maintain the same output luminance. The poor ageing performance is believed to be associated with the surface recombination of phosphors [1,2]. According to electron stimulated surface chemical reaction (ESSR) mechanism, the degradation is caused by the surface recombination of
⁎ Corresponding author. Present address: Department of Environmental Engineering, Beijing University of Chemical Technology, Beisanhuan Dong Lu 15, Beijing 100029, China. Tel.: +86 10 63651193; fax: +86 10 64713931. E-mail address: [email protected] (J. Yuan). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.11.015
phosphors (reaction with residue gases in the vacuum). Because of the existence of traces of oxygen, ZnS surfaces will be oxidized into ZnO dead layers which reduce cathodeluminescence (CL) intensity significantly. One way to slow the degradation rate is to coat phosphor surfaces with protective layers which can isolate the surfaces from the residue gases in the FED environment. Such coatings as In2O3 [3], MgO [4], TaSi2 [5], BaTiO3 [6] and calcium polyphosphate [7] have been investigated. However, none of those coatings is applicable. For example, In2O3 coatings can improve CL performance only at very low voltage (b 400 V) [3], the anti-degradation effect of calcium polyphosphate coatings is pronounced at midvoltage range (approximately 4 kV) [7], and aluminum compound coatings will cause a small change in color chromaticity [4]. Silica is expected to be one of the candidates for coatings because it has energy band gap larger than ZnS, and the wide band gap layer can reflect the electrons generated by the incident electron beam from the defective high surface recombination velocity region back into the phosphors [8]. In this paper, silica coatings on phosphors have been obtained via a sol–gel route using cetyltrimethylammonium chloride (CTAC) dispersant.
J. Yuan et al. / Materials Letters 60 (2006) 1284–1286
1285
2. Experimental ZnS-type phosphors used in this work are green emitting ZnS:Ag,Cl phosphors which mean particle size is 6.5 μm (Nichia Co., Japan). The silica coatings on phosphors are fabricated by sol–gel method using tetraethyl orthosilicate (TEOS) as the precursor. In a typical experiment, TEOS is first dissolved in the mixture of ethanol and distilled water, and pH value of the mixture is adjusted by diluted nitric acid. The solation process is conducted at 50–75 °C for 10 min. Together with the additive CTAC, a certain amount of green phosphors are suspended in distilled water and deagglomerated by treating for 5 min with ultrasonics. In order to get silica coatings on phosphors, the hydrolysis product is added to the suspension, and stirred for hours. The resulting product is separated from the solution, washed twice with ethanol and dried at 100–110 °C for 1 h, thereafter annealed at 400 °C in air for 1 h. Zeta potential of raw phosphors without and with the addition of CTAC is measured using Model 502 Zeta Potential Instrument (Rufuto Co. Ltd., Japan). Samples are dispersed into distilled and ion exchanged water, then pH of sample suspension is adjusted by adding very dilute hydrochloride solution or sodium hydroxide solution. In order to deagglomerate, sample suspension is treated by ultrasonication for 15 min. A JSM-5800 scanning electron microscopy (JEOL, Japan) and a HITACHI H-800 transmission electron microscopy (Hitachi Co., Japan) are used for microstructure observation, respectively. 3. Results and discussions Zeta potential of raw phosphors without and with the addition of CTAC as the function of pH is measured as shown in Fig. 1. This shows that the isoelectric point (IEP) of raw phosphors without the addition of CTAC is at pH ca. 3.1, which is different from that of ZnS phosphors in other literatures (pH ca. 7.0) [9,10]. It might be because the as-received commercial ZnS phosphors were already treated with particulate-like silica to improve their fluidity when making screens [11]. In our experiments, the coating process is conducted at pH 2–4, which is very near to IEP of raw phosphors. At the adjacency of IEP, phosphor powders are easy to agglomerate together because the
Fig. 1. Zeta potential of raw phosphors without and with the addition of CTAC as the function of pH.
Fig. 2. SEM images of uncoated (a) and SiO2 coated (b) phosphors.
repulsive force among phosphors is very weak. In order to increase the dispersibility of phosphors, CTAC, a cationic surfactant, is added into phosphor suspension. Because the special adsorption of CTAC molecules on the phosphors, the absolute value of zeta potential increases (Fig. 1), that is to say, the repulsive force among phosphors increase significantly at that dosage of CTAC. With the addition of CTAC, phosphor powders are very easy to disperse, which will promote the coating process. The scanning electron microscopy (SEM) images of uncoated and SiO2 coated phosphors are presented in Fig. 2. As shown in Fig. 2(a), there are many small spots on raw materials, which diameter is estimated to be about 200 nm. Compared with raw phosphors, the surfaces of coated phosphors look smoother because small spots on raw phosphors are partially covered with SiO2 coatings (Fig. 2(b)). The transmission electron microscopy (TEM) images of uncoated and SiO2 coated materials are presented in Fig. 3. Fig. 3 shows that there is a transparent uniform and continuous film on the coated phosphor compared with the raw phosphor. That coating is expected to inhibit the surface oxidation of ZnS. Merikhi and Feldmann reported that colloidal SiO2 particles adhere on ZnS-type phosphor surfaces [10]. Unfortunately, rather than uniform and continuous coatings, the phosphors are covered by island-like SiO2 particles, which cannot be expected to improve the ageing performance significantly because of uncompleted coverage. In literature [10], colloidal ZnO particles are first attached to phosphor surfaces, and colloidal SiO2 particles to ZnO particle surfaces by electrostatic attractive force, in which ZnO particles act as bridges between ZnS phosphors and colloidal SiO2 particles. Because phosphor surfaces are partially covered by ZnO particles, it is difficult
Fig. 3. TEM images of uncoated (a) and SiO2 coated (b) phosphors.
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to get complete coverage of SiO2. But in this study, raw phosphor surfaces are uniformly covered with CTAC molecules, thus the electric properties of the surfaces become more uniform, which will benefit the uniform gelation process of silicate acid sol on the whole surfaces. Furthermore, a uniform and continuous coating ascribes uniform particle surfaces and appropriate homogeneous nucleation rate. The gelation rate is slow enough to get a continuous film because of the precisely adjusted concentration of precursor solution in this study.
4. Conclusions The green emitting ZnS:Ag,Cl phosphors are covered with uniform and continuous SiO2 coatings by sol–gel method. The special adsorption of CTAC on raw phosphors increases the absolute value of zeta potential, which is helpful, not only for the dispersibility of phosphors but also for the formation of uniform and continuous coatings. Now we are trying to get thinner coatings with better ageing performance.
References [1] S. Itoh, T. Kimizuka, T. Tonegawa, J. Electrochem. Soc. 136 (1989) 1819. [2] K.T. Hillie, S.S. Basson, H.C. Swart, Appl. Surf. Sci. 187 (2002) 137. [3] H. Kominami, T. Nakamura, Y. Nakanishi, Y. Hatanaka, Jpn. J. Appl. Phys. 35 (1996) L1600. [4] R.Y. Lee, S.W. Kim, J. Lumin. 93 (2001) 93. [5] J.M. Fitz-Gerald, T.A. Trottier, R.K. Singh, P.H. Holloway, Appl. Phys. Lett. 72 (1998) 1838. [6] T. Nakamura, M. Kamiya, H. Watanabe, J. Electrochem. Soc. 142 (1995) 949. [7] H. Bechtel, W. Czarnojan, M. Haase, W. Mayr, H. Nikol, Philips J. Res. 50 (1996) 433. [8] W. Park, B.K. Wagner, G. Russell, K. Yasuda, C.J. Summers, Y.R. Do, H. G. Yang, J. Mater. Res. 11 (2000) 2288. [9] C. Feldmann, J. Merikhi, J. Colloid Interface Sci. 223 (2000) 229. [10] J. Merikhi, C. Feldmann, J. Colloid Interface Sci. 228 (2000) 121. [11] J.-C. Souriau, Y.D. Jiang, J. Penczek, H.G. Paris, C.J. Summers, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 76 (2000) 165.