pubs.acs.org/Langmuir © 2009 American Chemical Society
Reactive Formation of Zircon Inclusion Pigments by Deposition and Subsequent Annealing of a Zirconia and Silica Double Shell Feng Zhao,†,‡ Yanfeng Gao,*,† and Hongjie Luo† †
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Dingxi 1295, Shanghai, China, and ‡ Graduate School of the Chinese Academy of Sciences, Yuquanlu 19, Beijing 100049, China Received August 26, 2009. Revised Manuscript Received September 28, 2009
A novel general method for coating particles with a complex oxide was described. Zirconia precursor and silica layers with careful control of film thickness were coated separately onto hematite particles in corresponding solutions. A zircon shell was subsequently obtained by heat treatment at 800 °C for 3 h using LiF as a mineralizer. The as-prepared zirconoccluded hematite pigment gave a pink color to the glazed sample after annealing at 1120 °C. The current research suggests that various chromophoric particles can be encapsulated with zircon to prepare ceramic pigments for high-temperature use.
Core-shell structured particles on the scale of nanometers to micrometers have attracted intensive interest because of their variety of potential and practical applications in different fields, such as sensors,1 cosmetics,2 pharmaceutics,3,4 and pigments among others.5-7 Surface modification often provides particles with the properties required for such functions. These requirements may include anticorrosion, dispersivity, solubility, and colloidal stability.8 Among all of the inorganic oxides used as shells, silica has been extensively investigated not only because a silica shell is easily fabricated on various particles via the well-known St€ober method9,10 but also because the inert shell prevents the particles from degrading or aggregating in their application environment.11,12 However, for severe environments where silica cannot endure, some other materials must be investigated as alternatives. For example, ceramic pigments are used in glazes at elevated temperatures, which greatly restricts the applications of most pigments because of the resulting instability. Encapsulating these pigments into refractory materials can protect them from reaction with other components in glazes. Zircon (ZrSiO4)13 is usually selected as a refractory material because of its chemical stability, lack of effect in coloring, and crystallographic stability. Iron-pink and cadmium-sulfoselenide-red zircon pigments are representative inclusion pigments in which hematite or cadmium sulfosele*Corresponding author. E-mail:
[email protected]. Fax: þ86-21-52415270. (1) Chavez, J. L.; Wong, J. L.; Duran, R. S. Langmuir 2008, 24, 2064–2071. (2) Jaroenworaluck, A.; Sunsaneeyametha, W.; Kosachan, N.; Stevens, R. Surf. Interface Anal. 2006, 38, 473–477. (3) Soppimath, K. S.; Tan, D. C. W.; Yang, Y. Y. Adv. Mater. 2005, 17, 318–323. (4) Guo, M.; Yan, Y.; Zhang, H.; Yan, H.; Cao, Y.; Liu, K.; Wan, S.; Huang, J.; Yue, W. J. Mater. Chem. 2008, 18, 5104–5112. (5) Lin, J.; Yu, M.; Lin, C.; Liu, X. J. Phys. Chem. C 2007, 111, 5835–5845. (6) Lin, C. K.; Li, Y. Y.; Yu, M.; Yang, P. P.; Lin, J. Adv. Funct. Mater. 2007, 17, 1459–1465. (7) Yu, M.; Lin, J.; Fang, J. Chem. Mater. 2005, 17, 1783–1791. (8) Chen, Q.; Boothroyd, C.; Tan, G. H.; Sutanto, N.; Soutar, A. M.; Zengl, M. T. Langmuir 2008, 24, 650–653. (9) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329– 4335. (10) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693–6700. (11) Kobayashi, Y.; Horie, M.; Konno, M.; Rodriguez-Gonzalez, B.; Liz-Marzan, L. M. J. Phys. Chem. B 2003, 107, 7420–7425. (12) Jana, N. R.; Earhart, C.; Ying, J. Y. Chem. Mater. 2007, 19, 5074–5082. (13) Jansen, M.; Letschert, H. P. Nature 2000, 404, 980–982. (14) Carreto, E.; Pina, C.; Arriola, H.; Barahona, C.; Nava, N.; Castano, V. J. Radioanal. Nucl. Chem. 2001, 250, 453–458.
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nide particles are occluded in a zircon matrix to give a pink or red color.14-17 Among most processes, ferrous or ferric salts were often selected as starting materials to synthesize iron-inclusion pigments18,19 in which hematite particles were produced and occluded in zircon simultaneously. Recently, we reported a modified sol-gel method to prepare gray and pink pigments by dispersing carbon black or hematite powders into a zircon precursor gel.20 However, these processes can produce only zircon inclusion pigments in which the chromophoric particles are randomly dispersed in the zircon matrix, whereas many bare chromophoric particles remain unencapsulated. The poor encapsulation efficiency is the main shortcoming of traditional methods and is mainly due to the process in which zircon formation and chromophore encapsulation occur at the same time. To improve the encapsulation efficiency of the chromophore in zircon, a new strategy for zircon coating is proposed through fabricating chromophore-zircon core-shell particles directly in solution. However, a crystalline zircon shell is difficult to prepare in solution. Even a precursor mixture of silica and zirconia is not easy to precipitate from a one-pot solution because these two precursors have different formation conditions in terms of the degree of supersaturation.21 Hence, an optional process is proposed in which zirconia and silica layers are coated separately in corresponding solutions. A zircon shell is subsequently obtained by heat treatment. To ensure proper thickness and complete reaction of the zirconia shell with silica in the heating process, both the zirconia and silica shells should be controllable in terms of thickness. As for the zircon-formation process by heat treating zirconia and silica, it has been confirmed that Si4þ diffuses into zirconia sites to produce zircon,22,23 so a zirconia layer is coated first, followed by a silica layer. In this case, hematite particles were (15) Llusar, M.; Calbo, J.; Badenes, J. A.; Tena, M. A.; Monros, G. J. Mater. Sci. 2001, 36, 153–163. (16) Cappelletti, G.; Ardizzone, S.; Fermo, P.; Gilardoni, S. J. Eur. Ceram. Soc. 2005, 25, 911–917. (17) Lavilla, V. L.; Lopez, J. M. R. Trans. J. Br. Ceram. Soc. 1981, 80, 105–108. (18) Tartaj, P.; Gonzalez-Carreno, T.; Serna, C. J.; Ocana, M. J. Solid State Chem. 1997, 128, 102–108. (19) Cui, H. T.; Ren, W. Z. J. Non-Cryst. Solids 2008, 354, 5432–5434. (20) Zhao, F.; Li, W.; Luo, H. J. Sol-Gel Sci. Technol. 2009, 49, 247–252. (21) Itoh, T. J. Mater. Sci. Lett. 1994, 13, 1661–1663. (22) Eppler, R. A. J. Am. Ceram. Soc. 1970, 53, 457–462. (23) Veytizou, C.; Quinson, J. F.; Valfort, O.; Thomas, G. Solid State Ionics 2001, 139, 315–323.
Published on Web 10/29/2009
DOI: 10.1021/la903197t
13295
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Figure 1. TEM photographs of Fe2O3@ZrO2 particles prepared by the multistep deposition of zirconium sulfate: (a) once, (b), twice and (c) three times. Scheme 1. Synthesis Procedure of Zircon-Coated Hematite Particles
selected as the chromophore material. The synthesis procedure is illustrated in scheme 1. The first step is zirconia deposition. Most of the processes for ZrO2 deposition use zirconium alkoxides as starting materials,24,25 which are much more expensive than inorganic salts. Through the hydrolysis of a zirconium oxychloride solution, an ultrathin zirconia shell can be deposited, but it is difficult to increase the shell thickness using these processes.26,27 We recently reported a facile surface-coating method using zirconium sulfate as a starting material,28 which enabled us to tune the thickness of the zirconia precursor shell to within tens of nanometers using precursor chemistry combined with the regulation of solution conditions, such as pH and temperature. However, these treatments usually take a long time, resulting in the aggregation of the hematite particles. In this research, we further optimized the deposition process and developed a multistep deposition process by adding a defined amount of zirconium sulfate to the deposition solutions at the proper times. According to thermodynamic analysis,28 changes in the supersaturation degree and surface properties have important effects on the deposition process. The addition of fresh zirconium sulfate solution to the original reaction solution can regulate the degree of supersaturation; subsequently, precipitation of the zirconia precursor continues, resulting in a thick shell. However, the amount of fresh zirconium sulfate added should be optimized to induce precipitation on the particle surfaces while inhibiting homogeneous deposition in solution. Typically, 0.04 g of Fe2O3 particles were first dispersed in 90 mL of distilled water with 1 mL of 1 wt % hydroxypropyl cellulose (HPC) as dispersant. Then, 10 mL of a freshly prepared 0.02 M zirconium sulfate aqueous solution was added to the above solution. After hydrolysis at 30 °C for 2 h, the as-prepared zirconia shell was about 20 nm thick (sample A). In our process, a thick shell of about 45 nm (sample B) could be obtained by further (24) Chen, H. R.; Gao, J. H.; Ruan, M. L.; Shi, J. L.; Yan, D. S. Microporous Mesoporous Mater. 2004, 76, 209–213. (25) Chen, D.; Liu, J. S.; Wang, P.; Zhang, L.; Ren, J.; Tang, F. Q.; Wu, W. Colloids Surf., A 2007, 302, 461–466. (26) Zhao, F.; Gao, Y.; Li, W.; Luo, H. J. Ceram. Soc. Jpn. 2008, 116, 1164– 1166. (27) Lu, J.; Zang, J. B.; Shan, S. I.; Huang, H.; Wang, Y. H. Nano Lett. 2008, 8, 4070–4074. (28) Zhao, F.; Gao, Y.; Luo, H. Langmuir 2009, 25, 6940–6946.
13296 DOI: 10.1021/la903197t
Figure 2. (a) TEM photograph of Fe2O3@ZrO2@SiO2 particles. (b-d) EDS spectra of spots A-C, respectively, in the TEM photograph. The zirconia shell was prepared by the hydrolysis of a 2 mM zirconium sulfate solution at 30 °C for 2 h, and the silica shell was prepared by the hydrolysis of 1 mL of TEOS in 100 mL of an ethanol-water solution at 50 °C for 2 h.
addition of 10 mL of a 20 mM zirconium sulfate solution to the above solution with sonication, followed by hydrolysis under the same conditions for another 2 h. The zirconia shell was further increased to ∼70 nm in thickness (sample C) by repeating the addition and hydrolysis once a more time. The transmission electron microscopy (TEM, JEM-2010F, JEOL) images of samples A-C are respectively shown in Figure 1a-c. Silica layers were fabricated using the St€ober method in an ethanol-water solution of tetraethyl orthosilicate (TEOS).10,29 Typically, the as-prepared Fe2O3@ZrO2 particles were collected by centrifugation (4000 rpm) and subsequently dispersed in 100 mL of an ethanol-water solution (volume ratio E/W = 2.5), 1 mL of ammonia (25 wt %), and 1 mL of TEOS with sonication. The solution was then heated to 50 or 30 °C for 2 or 12 h, respectively. A TEM photograph (Figure 2a) of the asprepared particles clearly indicates that the hematite particles were coated with double shells. The inner and outside shells were about 13 and 15 nm in thickness, respectively. Energy-dispersive spectrometry (EDS) further confirmed the formation of two-shell coated particles, which were composed of zirconia and silica layers. Figure 2b suggests the coexistence of Si and Zr elements in the shell section. EDS results in Figure 2c,d clearly confirm the formation of ZrO2 precursor and SiO2 layers, as indicated by the TEM photograph in Figure 2a. The S and Cu peaks in the spectra (29) Han, K.; Zhao, Z. H.; Xiang, Z.; Wang, C. L.; Zhang, J. H.; Yang, B. Mater. Lett. 2007, 61, 363–368.
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Figure 3. TEM photograph of Fe2O3@ZrO2@SiO2 particles at (a) low and (b) high magnification, respectively. The process for depositing the zirconia shell was the same as that shown in Figure 1b, and the silica shell was prepared by the hydrolysis of 1 mL of TEOS in 100 mL of an ethanol-water solution at 30 °C for 12 h.
Figure 4. XRD patterns of Fe2O3@ZrO2@SiO2 powders after heat treatment at (a) 800, (b) 850, and (c) 900 °C for 3 h in air with 10 wt % LiF as a mineralizer.
are due to the sulfate ions in the zirconia precursor28 and sample support (copper grid), respectively. Furthermore, the thicknesses of both the silica and zirconia shells were also tunable under the optimized hydrolysis conditions. As shown in Figure 3, both shells are homogeneous and ∼40 nm thick. A zircon shell can result from the reaction of zirconia and silica layers with mineralizers such as alkali metal halides22,30 or alkaline earth halides,20 which can induce zircon formation at a low temperature (
