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Fabrication and characterization of silica nanocoatings on ZnS phosphors using sodium silicate as the precursor
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Materials Letters 62 (2008) 1782 – 1784 www.elsevier.com/locate/matlet
Fabrication and characterization of silica nanocoatings on ZnS phosphors using sodium silicate as the precursor Jiongliang Yuan a,⁎, Cunjiang Hao b , Zhimin Li a a
b
Department of Environmental Science and Engineering, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Beijing 100019, PR China Department of Experimental Chinese Medicine, Tianjin University of Traditional Chinese Medicine, 88 Yuquan Road, Nankai District, Tianjin 300193, PR China Received 4 July 2007; accepted 4 October 2007 Available online 18 October 2007
Abstract In order to prevent the surface oxidation of ZnS-type phosphors in field emission displays (FEDs), surface coating is an effective way. In this study, the uniform and continuous silica nanocoatings on ZnS-type phosphors are successfully obtained by the chemical precipitation method using sodium silicate as the precursor. The zeta potential and transmission electron microscopy (TEM) results show that ZnS phosphors are completely covered with uniform and continuous coatings. At pH 5.0 ± 0.5, the ZnS phosphors and silica powders carry opposite charges, thus attract each other easily. The resulted coatings are expected to inhibit the degradation of ZnS phosphors at higher current densities. © 2007 Elsevier B.V. All rights reserved. Keywords: Coatings; Phosphors; Silica; Chemical precipitation; Degradation
1. Introduction Recently the field emission displays (FEDs) have attracted increasing attention as a promising candidate for next-generation flat panel displays. But attempts worldwide to develop new, durable, and high-efficiency phosphors for FEDs have not yet been successful so far. ZnS-type phosphors used in cathode ray tubes (CRTs) have higher cathodoluminescent intensity; however, they are unsuitable for FEDs because of their fast degradation. FEDs operate at lower voltage excitation which requires higher current densities in order to maintain the same output luminance as in CRTs. Because of the existence of trace of oxygen in FED vacuum, the surface of ZnS phosphors can be oxidized into ZnO dead layer, which results in their fast degradation [1]. Surface coating is an effective way to prevent the surface oxidation. Such coatings as SiO2 [2–5], ZnO [6,7], TiO2 [8,9], In2O3 [10,11], and Y2O3 [12] have been investigated. However, none of those coatings has been put into practice so far. The qualified coatings must meet four basic requirements: complete coverage of the phosphors, optimal thickness of 5–10 nm, chemical and thermal stability, and no other luminescent centers ⁎ Corresponding author. Tel.: +86 10 63651193; fax: +86 10 64427356. E-mail address: [email protected] (J. Yuan). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.024
introduced. 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 [2]. The silica coatings can passivate surface recombination centers; therefore the surface recombination can be inhibited. Moreover, silica has a penetration depth nearly twice as much as ZnS and minimal loss in electron penetration [2]. There have already been some reports on silica coatings on ZnS phosphors so far [2,3,5]. In all of those reports, silica coatings were fabricated via sol–gel route using tetraethyl orthosilicate (TEOS) as the precursor [2,3,5]. However, the chemical precipitation method is developed to get silica coatings on ZnS phosphors using sodium silicate as the precursor in this study. 2. Experimental ZnS-type phosphors used here are green emitting ZnS:Ag,Cl phosphors with mean particle size of about 5.0 μm (Beijing Chemical Factory, PR China). The silica coatings on ZnS phosphors are fabricated by the chemical precipitation method using sodium silicate (Na2SiO3·9H2O, AR, Beijing Yili Refined Chemicals Co. Ltd., PR
J. Yuan et al. / Materials Letters 62 (2008) 1782–1784
China) as the precursor. In a typical experiment, ZnS phosphors are firstly dispersed into deionized water with ultrasonics, then 0.5 M sodium silicate solution, the dispersant and precursor, is introduced into the suspension. The resulted mixture is put into the water bath of 86 ± 2 °C and neutralized to pH 5.0 ± 0.5 by 0.01 M diluted hydrochloride acid solution under stirring. After neutralizing, the reaction product is aged for 2.0 h in the water bath, then filtered and washed. The coated phosphors are dried at 100–110 °C for 12 h, thereafter sintered at 500 °C in air for 4 h. Nanosized silica particles are fabricated at the same experimental condition. The zeta potentials of uncoated, coated phosphors and silica nanoparticles are measured using Zetasizer laser particle size machine (3000HS, Malvam, UK). The samples are dispersed into deionized water, and pH of samples suspension is adjusted by adding very dilute hydrochloride acid solution or sodium hydroxide solution. In order to deagglomerate, sample suspension is treated with ultrasonics for 15 min. A Hitachi H-800 transmission electron microscopy (TEM, Hitachi Co., Japan) is used for microstructure observation of the uncoated and coated phosphors. The Infra Red (IR) spectra of the silica powders and the coated phosphors are recorded in Nicolet 8700 Fourier Transform Infra Red (FT-IR) spectrometer in the range of 4000–400 cm− 1 as powders dispersed in KBr pellets. 3. Results and discussion The commercial phosphors used in the previous report [5] have already been treated with particulate-like silica. However, considering that the silica coatings will be formed, ZnS phosphors without pretreatment are used as raw materials in this study. The typical TEM images of the uncoated and silica-coated phosphors are presented in Fig. 1. It can be seen that the phosphor is covered with a uniform and continuous silica layer instead of island-like particulates (Fig. 1(b)). The coating is expected to inhibit the surface oxidation of ZnS phosphor. The zeta potentials for the uncoated, silica-coated phosphors and silica powders are plotted against the medium pH as shown in Fig. 2. It
Fig. 1. TEM images of the uncoated and silica-coated phosphors: (a) uncoated phosphor; (b) silica-coated phosphor.
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Fig. 2. Zeta potentials of the uncoated, silica-coated phosphors, and pure silica powders as the function of pH: (a) uncoated phosphors; (b) silica-coated phosphors; (c) pure silica powders.
is indicated that the iso-electric point (IEP) of the uncoated phosphors is at pH about 7.0 (Fig. 2a); however, it changes to pH about 2.0 for the silica-coated phosphors (Fig. 2b), the same as that pure silica powders (Fig. 2c). In addition, the zeta potential curve of the silica-coated behaves similar to that of pure silica powders, but quite different from that of the uncoated phosphors. Therefore, it can be followed that ZnS phosphors have completely been coated with silica. Fig. 3 shows the FT-IR spectra of the silica powders and the silicacoated phosphors. Since the absorption peak of ZnS occurs below 440 cm− 1, there is no special band for ZnS between 4000 cm− 1 and 500 cm− 1. It was reported that the stretch vibration peak of Si–O bond in amorphous silica locates at 1100–1010 cm− 1 [13]. In this study, the peak at 1100.1 cm− 1 should be corresponding to the stretch vibration peak of Si–O bond in pure silica powders (Fig. 3a). For the coated phosphors (Fig. 3b), although the weak absorption peak at 1089.6 cm− 1, adjacent to the stretch vibration peak of Si–O bond in pure silica powders, is not the minimum of the transmittance, it indicates the presence of amorphous silica. Because the amount of silica in coated phosphors is very small, the IR absorption should be similar to that of ZnS phosphors themselves. The broad absorption of the silica-coated phosphors in the range of wavenumber above 1200 cm− 1 is attributed to the phosphors themselves, which is similar to that of the titania-coated phosphors [9]. The pH value has a significant effect on the gelation rate of silicic acid. The gelation rate is slow enough at pH about 5.0 and 8.5 [14],
Fig. 3. FT-IR spectra of silica powders and the silica-coated phosphors: (a) pure silica powders; (b) silica-coated phosphors.
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J. Yuan et al. / Materials Letters 62 (2008) 1782–1784
which favors the formation of silica coatings. When the coating process is carried out at pH 2.0–4.0 or higher than 9.0, a large amount of silicic acid embryos are formed rapidly, thus individual silica particles occur in the solution rather than on phosphor surfaces. In our experiments, silica nanocoatings are successfully obtained at pH 5.0 ± 0.5; however, there is no silica coatings formed at pH 8.5 ± 0.5. This should be explained by the interaction between ZnS phosphors and silicic acid embryos. Since the IEP of ZnS phosphors and silica particles are at pH about 7.0 and 2.0, respectively, ZnS phosphors carry positive charges at pH 5.0 ± 0.5, while silicic acid embryos are negatively charged. Accordingly, the electrostatic attractive force occurs between ZnS phosphors and silicic acid embryos. As soon as silicic acid embryos are formed, they are easily adhered to phosphor surface and preferentially grow along the surface, therefore silica coatings can be obtained. However, both ZnS phosphors and silica particles are negatively charged at pH 8.5 ± 0.5, and the electrostatic repulsive force becomes dominant between ZnS phosphors and silicic acid embryos, thus silicic acid embryos are inclined to forming the individual particles in the solution instead of the continuous coatings on phosphor surfaces.
4. Conclusions The uniform and continuous silica nanocoatings are successfully obtained on ZnS-type phosphor surfaces by the chemical precipitation method using sodium silicate as the precursor. The coatings are formed when sodium silicate solution is neutralized to pH 5.0 ± 0.5 by diluted acid solution. At pH 5.0 ± 0.5, not only the electrostatic attractive force between ZnS phosphors and silicic acid embryos but also the appropriate gelation rate provides the opportunity for adhering silicic acid embryos to phosphor surface rather than forming individual silica particles in the solution. The zeta potential and TEM results show that ZnS phosphors are completely covered with silica coatings. The coatings are expected to inhibit the degradation of ZnS phosphors at higher current densities.
Acknowledgments The Project is sponsored by the National Natural Science Foundation of China (grant no.20776013), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SRF for ROCS, SEM) and the Natural Sciences Research Foundation for Young Teachers in Beijing University of Chemical Technology (grant no. QN0502). References [1] K.T. Hillie, S.S. Basson, H.C. Swart, Appl. Surf. Sci. 187 (2002) 137–144. [2] W. Park, B.K. Wagner, G. Russell, K. Yasuda, C.J. Summers, Y.R. Do, H.G. Yang, J. Mater. Res. 11 (2000) 2288–2291. [3] Y.R. Do, D.H. Park, H.G. Yang, W. Park, B.K. Wagner, K. Yasuda, C.J. Summers, J. Electrochem. Soc. 148 (2001) G548–G551. [4] J. Merikhi, C. Feldmann, J. Colloid Interface Sci. 228 (2000) 121–126. [5] J. Yuan, K. Kajiyoshi, K. Yanagisawa, H. Sasaoka, K. Nishimura, Mater. Lett. 60 (2006) 1284–1286. [6] T. Igarashi, T. Kusunoki, K. Ohno, T. Isobeb, M. Senna, Mater. Res. Bull. 36 (2001) 1317–1324. [7] C. Feldmamn, J. Merikhi, J. Colloid Interface Sci. 223 (2000) 229–234. [8] C. Guo, B. Chu, M. Wu, Q. Su, J. Lumin. 105 (2003) 121–126. [9] J. Yuan, D. Chen, M. Yang, P. Yue, Mater. Lett. 2007 (in press). [10] H. Kominami, T. Nakamura, K. Sowa, Y. Nakanishi, Y. Hatanaka, G. Shimaoka, Appl. Surf. Sci. 113/114 (1997) 519–522. [11] J.Y. Kim, D.Y. Jeon, L. Yu, H.G. Yang, J. Electrochem. Soc. 147 (2000) 3559–3563. [12] W. Park, K. Yasuda, B.K. Wagner, C.J. Summers, Y.R. Do, H.G. Yang, Mater. Sci. Eng. B76 (2000) 122–126. [13] G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed. Wiley, New York, 2001. [14] A. Cui, T. Wang, Y. Jin, Eng. Chem. & Metallurgy 10 (1999) 178–181 (in Chinese).
Materials Letters 62 (2008) 1782 – 1784 www.elsevier.com/locate/matlet
Fabrication and characterization of silica nanocoatings on ZnS phosphors using sodium silicate as the precursor Jiongliang Yuan a,⁎, Cunjiang Hao b , Zhimin Li a a
b
Department of Environmental Science and Engineering, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Beijing 100019, PR China Department of Experimental Chinese Medicine, Tianjin University of Traditional Chinese Medicine, 88 Yuquan Road, Nankai District, Tianjin 300193, PR China Received 4 July 2007; accepted 4 October 2007 Available online 18 October 2007
Abstract In order to prevent the surface oxidation of ZnS-type phosphors in field emission displays (FEDs), surface coating is an effective way. In this study, the uniform and continuous silica nanocoatings on ZnS-type phosphors are successfully obtained by the chemical precipitation method using sodium silicate as the precursor. The zeta potential and transmission electron microscopy (TEM) results show that ZnS phosphors are completely covered with uniform and continuous coatings. At pH 5.0 ± 0.5, the ZnS phosphors and silica powders carry opposite charges, thus attract each other easily. The resulted coatings are expected to inhibit the degradation of ZnS phosphors at higher current densities. © 2007 Elsevier B.V. All rights reserved. Keywords: Coatings; Phosphors; Silica; Chemical precipitation; Degradation
1. Introduction Recently the field emission displays (FEDs) have attracted increasing attention as a promising candidate for next-generation flat panel displays. But attempts worldwide to develop new, durable, and high-efficiency phosphors for FEDs have not yet been successful so far. ZnS-type phosphors used in cathode ray tubes (CRTs) have higher cathodoluminescent intensity; however, they are unsuitable for FEDs because of their fast degradation. FEDs operate at lower voltage excitation which requires higher current densities in order to maintain the same output luminance as in CRTs. Because of the existence of trace of oxygen in FED vacuum, the surface of ZnS phosphors can be oxidized into ZnO dead layer, which results in their fast degradation [1]. Surface coating is an effective way to prevent the surface oxidation. Such coatings as SiO2 [2–5], ZnO [6,7], TiO2 [8,9], In2O3 [10,11], and Y2O3 [12] have been investigated. However, none of those coatings has been put into practice so far. The qualified coatings must meet four basic requirements: complete coverage of the phosphors, optimal thickness of 5–10 nm, chemical and thermal stability, and no other luminescent centers ⁎ Corresponding author. Tel.: +86 10 63651193; fax: +86 10 64427356. E-mail address: [email protected] (J. Yuan). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.024
introduced. 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 [2]. The silica coatings can passivate surface recombination centers; therefore the surface recombination can be inhibited. Moreover, silica has a penetration depth nearly twice as much as ZnS and minimal loss in electron penetration [2]. There have already been some reports on silica coatings on ZnS phosphors so far [2,3,5]. In all of those reports, silica coatings were fabricated via sol–gel route using tetraethyl orthosilicate (TEOS) as the precursor [2,3,5]. However, the chemical precipitation method is developed to get silica coatings on ZnS phosphors using sodium silicate as the precursor in this study. 2. Experimental ZnS-type phosphors used here are green emitting ZnS:Ag,Cl phosphors with mean particle size of about 5.0 μm (Beijing Chemical Factory, PR China). The silica coatings on ZnS phosphors are fabricated by the chemical precipitation method using sodium silicate (Na2SiO3·9H2O, AR, Beijing Yili Refined Chemicals Co. Ltd., PR
J. Yuan et al. / Materials Letters 62 (2008) 1782–1784
China) as the precursor. In a typical experiment, ZnS phosphors are firstly dispersed into deionized water with ultrasonics, then 0.5 M sodium silicate solution, the dispersant and precursor, is introduced into the suspension. The resulted mixture is put into the water bath of 86 ± 2 °C and neutralized to pH 5.0 ± 0.5 by 0.01 M diluted hydrochloride acid solution under stirring. After neutralizing, the reaction product is aged for 2.0 h in the water bath, then filtered and washed. The coated phosphors are dried at 100–110 °C for 12 h, thereafter sintered at 500 °C in air for 4 h. Nanosized silica particles are fabricated at the same experimental condition. The zeta potentials of uncoated, coated phosphors and silica nanoparticles are measured using Zetasizer laser particle size machine (3000HS, Malvam, UK). The samples are dispersed into deionized water, and pH of samples suspension is adjusted by adding very dilute hydrochloride acid solution or sodium hydroxide solution. In order to deagglomerate, sample suspension is treated with ultrasonics for 15 min. A Hitachi H-800 transmission electron microscopy (TEM, Hitachi Co., Japan) is used for microstructure observation of the uncoated and coated phosphors. The Infra Red (IR) spectra of the silica powders and the coated phosphors are recorded in Nicolet 8700 Fourier Transform Infra Red (FT-IR) spectrometer in the range of 4000–400 cm− 1 as powders dispersed in KBr pellets. 3. Results and discussion The commercial phosphors used in the previous report [5] have already been treated with particulate-like silica. However, considering that the silica coatings will be formed, ZnS phosphors without pretreatment are used as raw materials in this study. The typical TEM images of the uncoated and silica-coated phosphors are presented in Fig. 1. It can be seen that the phosphor is covered with a uniform and continuous silica layer instead of island-like particulates (Fig. 1(b)). The coating is expected to inhibit the surface oxidation of ZnS phosphor. The zeta potentials for the uncoated, silica-coated phosphors and silica powders are plotted against the medium pH as shown in Fig. 2. It
Fig. 1. TEM images of the uncoated and silica-coated phosphors: (a) uncoated phosphor; (b) silica-coated phosphor.
1783
Fig. 2. Zeta potentials of the uncoated, silica-coated phosphors, and pure silica powders as the function of pH: (a) uncoated phosphors; (b) silica-coated phosphors; (c) pure silica powders.
is indicated that the iso-electric point (IEP) of the uncoated phosphors is at pH about 7.0 (Fig. 2a); however, it changes to pH about 2.0 for the silica-coated phosphors (Fig. 2b), the same as that pure silica powders (Fig. 2c). In addition, the zeta potential curve of the silica-coated behaves similar to that of pure silica powders, but quite different from that of the uncoated phosphors. Therefore, it can be followed that ZnS phosphors have completely been coated with silica. Fig. 3 shows the FT-IR spectra of the silica powders and the silicacoated phosphors. Since the absorption peak of ZnS occurs below 440 cm− 1, there is no special band for ZnS between 4000 cm− 1 and 500 cm− 1. It was reported that the stretch vibration peak of Si–O bond in amorphous silica locates at 1100–1010 cm− 1 [13]. In this study, the peak at 1100.1 cm− 1 should be corresponding to the stretch vibration peak of Si–O bond in pure silica powders (Fig. 3a). For the coated phosphors (Fig. 3b), although the weak absorption peak at 1089.6 cm− 1, adjacent to the stretch vibration peak of Si–O bond in pure silica powders, is not the minimum of the transmittance, it indicates the presence of amorphous silica. Because the amount of silica in coated phosphors is very small, the IR absorption should be similar to that of ZnS phosphors themselves. The broad absorption of the silica-coated phosphors in the range of wavenumber above 1200 cm− 1 is attributed to the phosphors themselves, which is similar to that of the titania-coated phosphors [9]. The pH value has a significant effect on the gelation rate of silicic acid. The gelation rate is slow enough at pH about 5.0 and 8.5 [14],
Fig. 3. FT-IR spectra of silica powders and the silica-coated phosphors: (a) pure silica powders; (b) silica-coated phosphors.
1784
J. Yuan et al. / Materials Letters 62 (2008) 1782–1784
which favors the formation of silica coatings. When the coating process is carried out at pH 2.0–4.0 or higher than 9.0, a large amount of silicic acid embryos are formed rapidly, thus individual silica particles occur in the solution rather than on phosphor surfaces. In our experiments, silica nanocoatings are successfully obtained at pH 5.0 ± 0.5; however, there is no silica coatings formed at pH 8.5 ± 0.5. This should be explained by the interaction between ZnS phosphors and silicic acid embryos. Since the IEP of ZnS phosphors and silica particles are at pH about 7.0 and 2.0, respectively, ZnS phosphors carry positive charges at pH 5.0 ± 0.5, while silicic acid embryos are negatively charged. Accordingly, the electrostatic attractive force occurs between ZnS phosphors and silicic acid embryos. As soon as silicic acid embryos are formed, they are easily adhered to phosphor surface and preferentially grow along the surface, therefore silica coatings can be obtained. However, both ZnS phosphors and silica particles are negatively charged at pH 8.5 ± 0.5, and the electrostatic repulsive force becomes dominant between ZnS phosphors and silicic acid embryos, thus silicic acid embryos are inclined to forming the individual particles in the solution instead of the continuous coatings on phosphor surfaces.
4. Conclusions The uniform and continuous silica nanocoatings are successfully obtained on ZnS-type phosphor surfaces by the chemical precipitation method using sodium silicate as the precursor. The coatings are formed when sodium silicate solution is neutralized to pH 5.0 ± 0.5 by diluted acid solution. At pH 5.0 ± 0.5, not only the electrostatic attractive force between ZnS phosphors and silicic acid embryos but also the appropriate gelation rate provides the opportunity for adhering silicic acid embryos to phosphor surface rather than forming individual silica particles in the solution. The zeta potential and TEM results show that ZnS phosphors are completely covered with silica coatings. The coatings are expected to inhibit the degradation of ZnS phosphors at higher current densities.
Acknowledgments The Project is sponsored by the National Natural Science Foundation of China (grant no.20776013), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SRF for ROCS, SEM) and the Natural Sciences Research Foundation for Young Teachers in Beijing University of Chemical Technology (grant no. QN0502). References [1] K.T. Hillie, S.S. Basson, H.C. Swart, Appl. Surf. Sci. 187 (2002) 137–144. [2] W. Park, B.K. Wagner, G. Russell, K. Yasuda, C.J. Summers, Y.R. Do, H.G. Yang, J. Mater. Res. 11 (2000) 2288–2291. [3] Y.R. Do, D.H. Park, H.G. Yang, W. Park, B.K. Wagner, K. Yasuda, C.J. Summers, J. Electrochem. Soc. 148 (2001) G548–G551. [4] J. Merikhi, C. Feldmann, J. Colloid Interface Sci. 228 (2000) 121–126. [5] J. Yuan, K. Kajiyoshi, K. Yanagisawa, H. Sasaoka, K. Nishimura, Mater. Lett. 60 (2006) 1284–1286. [6] T. Igarashi, T. Kusunoki, K. Ohno, T. Isobeb, M. Senna, Mater. Res. Bull. 36 (2001) 1317–1324. [7] C. Feldmamn, J. Merikhi, J. Colloid Interface Sci. 223 (2000) 229–234. [8] C. Guo, B. Chu, M. Wu, Q. Su, J. Lumin. 105 (2003) 121–126. [9] J. Yuan, D. Chen, M. Yang, P. Yue, Mater. Lett. 2007 (in press). [10] H. Kominami, T. Nakamura, K. Sowa, Y. Nakanishi, Y. Hatanaka, G. Shimaoka, Appl. Surf. Sci. 113/114 (1997) 519–522. [11] J.Y. Kim, D.Y. Jeon, L. Yu, H.G. Yang, J. Electrochem. Soc. 147 (2000) 3559–3563. [12] W. Park, K. Yasuda, B.K. Wagner, C.J. Summers, Y.R. Do, H.G. Yang, Mater. Sci. Eng. B76 (2000) 122–126. [13] G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed. Wiley, New York, 2001. [14] A. Cui, T. Wang, Y. Jin, Eng. Chem. & Metallurgy 10 (1999) 178–181 (in Chinese).