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Effects of coating parameters on the morphology of SiO2-coated TiO2 and the pigmentary properties
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Applied Surface Science 254 (2008) 2809–2819 www.elsevier.com/locate/apsusc
Effects of coating parameters on the morphology of SiO2-coated TiO2 and the pigmentary properties Yumin Liu, Chen Ge, Min Ren, Hengbo Yin *, Aili Wang, Dongzhi Zhang, Chunyan Liu, Jun Chen, Hui Feng, Hengping Yao, Tingshun Jiang Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China Received 4 October 2007; received in revised form 11 October 2007; accepted 11 October 2007 Available online 18 October 2007
Abstract SiO2-coated TiO2 powders were prepared by the chemical deposition method starting from rutile TiO2 and Na2SiO3. The SiO2-coated TiO2 powders were characterized by X-ray photoelectron spectroscopy, Zeta-potential analysis, Fourier transform infrared spectroscopy, and transmission electron microscopy. The evolution of island-like and uniform coating layers was found to depend upon the ratio of Na2SiO3 to TiO2, reaction temperature, and pH. The whiteness and brightness of the SiO2-coated TiO2 powders increased in response to an increase in the SiO2 loading, but there was a maximum value among the light scattering indexes. The SiO2-coated TiO2 powders possessed more negative Zeta potentials than the naked TiO2. The dispersibility of the SiO2-coated TiO2 powders with the continuous and uniform SiO2 coating layers was higher than that of the naked TiO2 and the SiO2-coated TiO2 powders with the island-like SiO2 coating layers. # 2007 Elsevier B.V. All rights reserved. Keywords: Rutile TiO2; SiO2; Island-like coating; Uniform coating; Pigmentary property
1. Introduction Commercial TiO2 powders with a particle size of more than 100 nm are widely used as a white pigment in paints, plastics, and the paper industry on account of its nontoxicity, its opacity, its high chemical stability, and its excellent whiteness and brightness. Since TiO2 has a strong photocatalytic activity, the covering of the TiO2 surface with nonactive oxides, such as SiO2, Al2O3, and ZrO2, is important in suppressing its photoactivity and improving its weather durability [1–3]. Rutile TiO2 powders are industrially produced by two methods: chloride and sulfate. The chloride method is via the gas phase reaction of TiCl4 with O2 at 1700 8C to produce TiO2 powders directly. The sulfate method is via the calcination (at ca. 1000 8C) of the precipitates produced by hydrolysis of titanyl sulfate. Rutile TiO2 powders produced by either method must then be coated with protective layers of SiO2 and A12O3 through wet chemistry processes in order to decrease their
* Corresponding author. Tel.: +86 511 88787591. E-mail address: [email protected] (H. Yin). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.10.021
photoactivity, to increase their weather durability, and to increase their dispersibility in liquids [1–3]. The rutile TiO2 powders produced by the chloride method have a narrower particle size distribution and a smoother surface than that produced by the sulfate method. After covering the inert layer on the TiO2 surface, the former has better pigmentary properties than the latter. Although several patents dealt with the covering of the rutile TiO2 powders in order to improve its pigmentary properties and its weather durability [4–6], few papers discuss in great detail the effects of the experimental conditions on the coating process of the inert layers and the effected optical properties [3]. In our present study, we investigated the effects of such coating parameters as reaction temperature, pH, and SiO2 loading. We examined the morphology of the SiO2 layer that forms on the surface of rutile TiO2 that was produced by the sulfate method. The experimental results show that both island-like and uniform coating layers can be manipulatively achieved by changing these experimental parameters. The coating was observed by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy.
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2. Experimental 2.1. Materials Rutile TiO2 powders with an average particle size of 300 nm were supplied by Zhenjiang Taibai Company as a gift. The rutile TiO2 powders were produced by the sulfate method without coating. The other chemicals, such as Na2SiO3, sulfuric acid, and sodium hydroxide, were of reagent grade and were used without further purification. Distilled water was used throughout all of the experiments.
for 20 min prior to the measurement. Zeta-potential measurement of the samples was conducted on a Zeta-potential meter (BDL-B). 0.2 g of the samples was dispersed in 200 ml of distilled water and the suspension was ultrasonicated for 20 min prior to Zeta-potential measurement. The brightness, whiteness, and light scattering index of the samples were analyzed using a spectrophotometer (CM-2500d), Minolta Co. Ltd., XL-30, and D65 illuminant. 3. Results and discussion 3.1. Effect of reaction temperature on the covering extent
2.2. Sample preparation Seventy-five grams of rutile TiO2 powder were ultrasonically treated in 750 ml of water for 20 min to obtain a welldispersed TiO2 suspension. The TiO2 suspension was transferred into a 2000 ml flask, kept in a water bath, and heated to a prescribed temperature. The TiO2 suspension was adjusted to a specific pH value by adding a NaOH (0.5 mol/l) aqueous solution. Aqueous solutions of Na2SiO3 (0.5 mol/l) and H2SO4 (0.1 mol/l) were added to the TiO2 suspension with two constant flow pumps. The flow rate of the Na2SiO3 aqueous solution was kept constant and the feeding time was fixed at 3 h. The pH value of the reaction solution was kept constant during the coating process by adjusting the flow rate of the sulfuric acid. After feeding, the resultant suspension was aged for 2 h at the specific pH. The precipitate was filtrated and washed with distilled water until the conductivity of the filtrate was less than 20 mS/m. Then the washed precipitate was dried in an electric oven at 105 8C for 2 h. The dried samples were kept in a desiccator for characterization. 2.3. Characterization The as-prepared samples were observed by transmission electron microscopy (TEM, Philips Tecnnai-12, operating at 120 kV) to investigate the effects of the coating parameters on the morphology and the thickness of the coating layer of the SiO2coated TiO2 powders. The samples for TEM were prepared by dispersing a small amount of the SiO2-coated TiO2 powders in ethanol and ultrasonicated for 10 min. Then a few drops of the resultant suspension were dropped onto a copper grid coated with a layer of amorphous carbon. The Fourier transform infrared spectra of the SiO2-coated TiO2 powders were performed by the KBr pellet technique on a Fourier transform infrared spectrometer (Nicolet Nexus470) to determine the interfacial chemical bonding structure. XPS analysis was carried out on a VG ESCALB spectrometer equipped with Mg Ka X-ray source, operating at 300 W. For all the samples, the spectra of C 1s, Si 2p, O 1s, and Ti 2p were recorded. The binding energies were referred to the C 1s binding energy at 284.5 eV. The average particle size and the size distribution of the samples were determined by a particle size distribution analyzer (BIC-90 Zeta pals, Brookhaven Instruments Inc.) with Dynamic Laser Scattering (DLS) mode. 0.1 g of the samples was dispersed in 150 ml of distilled water and the suspension was ultrasonicated
Fig. 1 shows the TEM images of the naked TiO2 and the SiO2-coated TiO2 powders prepared at different reaction temperatures. The pH value of the reaction solution was 9.5 and the mole ratio of Na2SiO3 to TiO2 was 1:25. As certified by the TEM image, the TiO2 surface was not coated by SiO2 when the reaction temperature was 30 8C. When the reaction temperature was raised to 50 8C, the loosely islandlike SiO2 coating layer formed on the TiO2 surface. The average particle size of the SiO2 particles (the islands), parallel to TiO2 surface, was ca. 7.5 nm and the average thickness of the SiO2 coating layers was ca. 4 nm. Raising the reaction temperature to 80 8C caused a denser island-like SiO2 coating layer to form. The average particle size of the SiO2 particles increased to approximately 9.5 nm and the average thickness of the islandlike SiO2 coating layers increased to ca. 5 nm. Further raising the reaction temperature to 90 8C increased the average particle size of the denser island-like SiO2 coating layer to 13 nm. The densely continuous and uniform SiO2 coating layers formed with an average thickness of 5 nm. Therefore, it can be concluded that raising the temperature of this reaction causes the formation of a dense SiO2 coating layer on the surface of the rutile TiO2 powder. 3.2. Effects of pH and Na2SiO3 content on the morphology of SiO2 coating layer To investigate the effects of the pH value of the reaction solution and the Na2SiO3 content on the morphology of the SiO2 coating layer on the TiO2 surface, the SiO2-coated TiO2 samples were prepared at a given reaction temperature of 90 8C, the mole ratios of Na2SiO3 to TiO2 were 1:25, 1:15, and 1:7.5, respectively, and the pH values of the reaction solutions were changed from 7 to 11. Fig. 2 shows the TEM images of the SiO2-coated TiO2 samples prepared with a given mole ratio of Na2SiO3 to TiO2 of 1:25 at the different pH values of the reaction solutions. From the TEM images, it can be found that the naked TiO2 powders have smooth surfaces and the average primary particle size is ca. 300 nm. When the covering process was carried out at the pH values ranging from 7 to 9, the SiO2 coating layers were found to be composed of SiO2 particles with an average particle size of ca. 15 nm. The average thickness of the resultant islandlike coating layers was ca. 8 nm. When the pH value was increased to 9.5, the surface of the TiO2 was partially covered
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Fig. 1. TEM images of the SiO2-coated TiO2 samples prepared at a pH value of 9.5 and different reaction temperatures with a mole ratio of Na2SiO3 to TiO2 of 1:25.
by the densely continuous and uniform coating layers with an average thickness of 5 nm. Also, the island-like coating layers formed with an average particle size of ca. 13 nm. When the pH value was further increased to 10, only a trace of the densely uniform coating layer was found on the TiO2 surface. When the pH value was more than 10, the surface of the TiO2 powders was the same as that of the naked TiO2 powders. There was no SiO2 coating layer found on the TiO2 powder surface. The TEM images of the SiO2-coated TiO2 samples prepared at the different pH values are shown in Fig. 3 for a mole ratio of Na2SiO3 to TiO2 of 1:15. As the pH values ranged from 7 to 8.5, the densely island-like SiO2 coating layers formed. The average particle size of the SiO2 particles was ca. 18 nm and the thickness of the densely island-like coating layers was ca. 8 nm. When the pH values were increased to 9 and 10, the densely continuous and uniform SiO2 coating layers were dominantly formed and the average thicknesses of the SiO2 coating layers were 8 and 5 nm, respectively. When the pH value of the
reaction solution was more than 10, a SiO2 coating layer was not formed on the TiO2 surface. As the mole ratio of Na2SiO3 to TiO2 was increased to 1:7.5, TEM images show that the densely island-like SiO2 coating layers were dominantly formed when the pH value of the reaction solution was 7 (Fig. 4). The average particle size of the SiO2 particles was 20 nm and the average thickness of the SiO2 coating layers was 13 nm. While increasing the pH values from 7 to 8, 8.5 and 9, the morphologies of the SiO2 coating layers changed from densely island-like coating to densely continuous and uniform coating. When the pH values were 9–10, the densely continuous and uniform coating layers were dominantly formed with an average thickness of ca. 11 nm. When the pH value reached 10.5, the covering extent was low. There was no SiO2 coating layer found when the pH value was more than 10.5. The results show that the thickness of the SiO2 coating layer was increased by increasing the mole ratio of Na2SiO3 to TiO2
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Fig. 2. TEM images of the SiO2-coated TiO2 samples prepared at a reaction temperature of 90 8C and different pH values with a mole ratio of Na2SiO3 to TiO2 of 1:25.
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Fig. 3. TEM images of the SiO2-coated TiO2 samples prepared at a reaction temperature of 90 8C and different pH values with a mole ratio of Na2SiO3 to TiO2 of 1:15.
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Fig. 4. TEM images of the SiO2-coated TiO2 samples prepared at a reaction temperature of 90 8C and different pH values with a mole ratio of Na2SiO3 to TiO2 of 1:7.5.
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Fig. 4. (Continued ).
Fig. 5. FTIR spectra of the SiO2-coated TiO2 samples prepared at a pH value of 9.5 and a reaction temperature of 90 8C with different mole ratios of Na2SiO3 to TiO2: (a) naked TiO2; (b)–(e) the mole ratios of Na2SiO3 to TiO2 were 1:75, 1:25, 1:15, and 1:7.5, respectively.
Fig. 6. X-ray photoelectron spectra of O 1s of the samples: (a) naked TiO2 and (b)–(d) samples prepared at a reaction temperature of 90 8C and a pH value of 9.5 with different mole ratios of Na2SiO3 to TiO2 (b, 1:75; c, 1:25; d, 1:7.5).
Fig. 7. X-ray photoelectron spectra of Ti 2p of the samples: (a) naked TiO2 and (b)–(d) samples prepared at a reaction temperature of 90 8C and a pH value of 9.5 with different mole ratios of Na2SiO3 to TiO2 (b, 1:75; c, 1:25; d, 1:7.5).
Fig. 8. X-ray photoelectron spectra of Si 2p of the samples prepared at a reaction temperature of 90 8C and a pH value of 9.5 with different mole ratios of Na2SiO3 to TiO2 (b, 1:75; c, 1:25; d, 1:7.5).
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and that the densely continuous and uniform SiO2 coating layers formed in a large range of pH values when the mole ratio of Na2SiO3 to TiO2 was high. On the other hand, when the pH value of the reaction solution was more than 10.5, SiO2 coating layers were not formed on the surface of the TiO2 powders. 3.3. IR analysis Fig. 5 shows the FT-IR spectra of the naked TiO2 powders and the SiO2-coated TiO2 samples prepared at a pH value of 9.5 and a reaction temperature of 90 8C with the different mole ratios of Na2SiO3 to TiO2. The bands at 800–400 cm 1 assigned to Ti–O–Ti vibrations were observed over all of the samples. The band at ca. 955.73 cm 1 occurred only in the spectra of the SiO2-coated TiO2 samples, indicating that SiO2 coating layers were anchored on the TiO2 surface by the formation of Ti–O–Si bonds [7,8]. While increasing the mole ratio of Na2SiO3 to TiO2, the intensity of the band at 955.73 cm 1 decreased due to the thicker SiO2 coating layer. Furthermore, the bands at 1089.24 and 1231.39 cm 1 assigned to the asymmetric stretching vibration modes of the Si–O–Si bridge [8,9] were observed when the mole ratio of Na2SiO3 to TiO2 was 1:25. The band intensities increased when the mole ratio of Na2SiO3 to TiO2 increased, revealing that the thickness of the SiO2 coating layer increased along with the SiO2 loading. 3.4. XPS analysis Fig. 6 shows the O 1s peaks of the naked rutile TiO2 and SiO2-coated TiO2 samples. The binding energy of O 1s peak of the naked rutile TiO2 was 529.1 eV. When the SiO2-coated TiO2 samples were prepared with the mole ratios of Na2SiO3 to TiO2 of 1:75, 1:25, and 1:7.5, the binding energies of O 1s peaks were 529.4, 532.25, and 532.65 eV, respectively. The O 1s peak located at around 529 eV should be ascribed to that of rutile TiO2 and the O 1s peak at around 532 eV ascribed to that of SiO2. The shift of the O 1s peaks certifies that SiO2 reacted on the TiO2 surface through the Ti–O–Si chemical bond at the interface of the SiO2 coating layer and the TiO2 particle, which is consistent with the IR analysis. The binding energy of Ti 2p3/2 of the naked rutile TiO2 was 457.65 eV (Fig. 7). When the SiO2-coated TiO2 samples were prepared with the mole ratios of Na2SiO3 to TiO2 of 1:75, 1:25, and 1:7.5, the binding energies of the Ti 2p3/2 and Si 2p of the samples were 457.95, 101.65; 458.3, 103; and 458.3, 103.45 eV, respectively (Figs. 7 and 8). While increasing SiO2 loading, the shifts of the Ti 2p3/2 and Si 2p peaks toward high binding energy reveal that the chemical states of Si and Ti changed. When the mole ratios of Na2SiO3 to TiO2 were increased to 1:25 and 1:7.5, the binding energies of the Ti 2p3/2 for both samples remained the same, indicating that the chemical state of Ti atoms at the interface was not affected by further increasing the SiO2 loading, i.e. the reaction between TiO2 surface and the SiO2 coating layer was complete. This result also indicates that the TiO2 surface was completely covered by SiO2 coating layers when the mole ratio of Na2SiO3 to TiO2 was increased to 1:25. On the other hand, the binding
energies of Si 2p ceaselessly increased and reached that of bulk SiO2 (103.35 eV) [10] with the increasing of the SiO2 loading. From the TEM analysis, it was found that the SiO2 coating layer existed in both densely island-like and uniform coating styles on the TiO2 surface when the mole ratio of Na2SiO3 to TiO2 was 1:25 and that the SiO2 coating layer dominantly existed in the uniform coating style when the mole ratio of Na2SiO3 to TiO2 was increased to 1:7.5. Therefore, it is reasonable to conclude that the morphology of the SiO2 coating layer affects the chemical state of Si atom in addition to the chemical bond between the Si of a SiO2 coating layer and the Ti of TiO2 via O atom at their interface. On the other hand, other researchers reported that SiO2 coating on TiO2 was still amorphous even when calcined at a high temperature of up to 1100 8C [11]. Therefore, the SiO2 coating layer formed under our present experimental conditions should exist in amorphous phase. 3.5. Coating process of SiO2 layer The isoelectric point (IEP) of rutile TiO2 is around pH 3.5 [3,12]. Under the present experimental conditions, the pH value of the reaction solution was greater or equal to 7; the rutile TiO2 surface was negatively charged. When the pH values of the reaction solutions were in a range of 7–8, Na2SiO3 was rapidly hydrolyzed to form a large number of siliceous micelles. The resultant siliceous micelles were anchored on the surface of TiO2 powders via Ti–O–Si bonding to obtain the island-like coating layers. Increasing the pH value to 9–10 should lower the hydrolysis rate of Na2SiO3 resulting in the formation of small-sized or less aggregated siliceous micelles. First, the small-sized micelles were anchored on the surface of TiO2 powders via Ti–O–Si bonding. Then both the micelles present in the solution and the anchored micelles condensed via Si–O–Si bonding to obtain continuous and uniform coating layers. When the pH value was more than 10.5, silicon species exist as single silicate anions, or less aggregated
Fig. 9. Zeta potentials of the SiO2-coated TiO2 samples prepared at a pH value of 9.5 and a reaction temperature of 90 8C with different mole ratios of Na2SiO3 to TiO2.
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siliceous micelles with quite small particle size, which should be more negatively charged. The single silicate anions and the highly negatively charged siliceous micelles did not react with the negatively charged TiO2 surface due to the strong electrostatic repulsion. Therefore, no SiO2 coating layers were formed on the TiO2 surface at this higher pH value. 3.6. Zeta-potential measurement The Zeta potentials of the naked TiO2 and the SiO2-coated TiO2 samples are shown in Fig. 9. After coating SiO2, the Zeta
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potentials rapidly decreased from 24 mVof the naked TiO2 to ca. 50 mVof the SiO2-coated TiO2 samples, revealing that the SiO2-coated TiO2 samples were more negatively charged than the naked TiO2. The Zeta potentials of the SiO2-coated TiO2 samples roughly kept constant while varying the mole ratio of Na2SiO3 to TiO2 from 1:75 to 1:7.5. The results indicate that the charging property of the TiO2 surface was significantly affected by the SiO2 coating rather than the thickness of the SiO2 coating layers. Furthermore, the more negatively charged SiO2-coated TiO2 powders with higher electrostatic repulsion will result in finer particle dispersibility.
Fig. 10. Particle size distributions of the samples prepared at a reaction temperature of 90 8C and different pH values with a mole ratio of Na2SiO3 to TiO2 of 1:15.
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Fig. 11. Particle size distributions of the samples prepared at a reaction temperature of 90 8C and a pH value of 9.5 with different mole ratios of Na2SiO3 to TiO2.
3.7. Effects of pH and Na2SiO3 content on the dispersibility of SiO2-coated TiO2 Fig. 10 shows that the particle size distribution of the naked TiO2 powders was classified into two categories: the primary particles had an average particle size of 250 nm with a particle number percentage of 87% and the secondary particles had an average particle size of 800 nm with a particle number percentage of 13%. For the SiO2-coated TiO2 samples prepared with a mole ratio of Na2SiO3 to TiO2 of 1:15 at pH values of 7, 8.5, 9, 9.5, and 10, the particle size distributions were also classified into two categories (Fig. 10): the average particle sizes and the particle number percentages of the primary particles were 250 nm, 75%; 250 nm, 76%; 300 nm, 98%; 300 nm, 100%; and 300 nm, 95%, respectively, and the average particle sizes and the particle number percentages of the secondary particles were 800 nm, 25%; 800 nm, 24%; 800 nm, 2%; 800 nm, 0%; and 900 nm, 5%. When the samples were prepared at the high pH values between 9 and 10, the dispersibilities of the resultant samples were higher than that of the naked TiO2. As certified by TEM analysis, the island-like coating layers formed at the pH values between 7 and 8.5, and the continuous and uniform coating layers formed at the pH values between 9 and 10. Therefore, it is reasonable to conclude that the formation of the continuous and uniform coating is beneficial to the dispersion of the SiO2-coated TiO2 powders in water. To investigate the effect of the SiO2 loading on the dispersibility of the SiO2-coated TiO2 powders in water, the
particle size distributions of the samples prepared at a pH value of 9.5 with the mole ratios of Na2SiO3 to TiO2 of 1:25 and 1:7.5 were also determined by DLS technique, respectively (Fig. 11). The average particle sizes and the particle number percentages of the primary particles were 300 nm, 96%; and 300 nm, 94%, respectively, and the average particle sizes and the particle number percentages of the secondary particles were 1000 nm, 4%; and 900 nm, 6%. Comparing the dispersibilities of the naked TiO2 and the samples prepared with the mole ratios of Na2SiO3 to TiO2 of 1:25, 1:15, and 1:7.5, we found that the well-dispersed SiO2-coated TiO2 powders were obtained when the mole ratio of Na2SiO3 to TiO2 was 1:15. 3.8. The pigmentary property of the SiO2-coated TiO2 sample According to Table 1, it was found that the whiteness and brightness of the SiO2-coated TiO2 samples increased in response to an increase in the SiO2 loading. This is because of the colored metal impurity, such as with Fe3+ ions, present in the naked TiO2. The surface was further hidden with the increase of the SiO2 loading. On the other hand, the relative light scattering index had a maximum value with the increase of the SiO2 loading. It is commonly believed that the relative light scattering index is affected by the dispersibility and the intrinsic properties of the particles. The presence of the maximum relative light scattering index has two explanations: (1) there was an optimum value for SiO2 loading to obtain the well-dispersed SiO2-coated TiO2 powders and (2)
Table 1 Color schemes of SiO2-coated TiO2 samples (in system CIE) Samples
Mole ratios of Na2SiO3 to TiO2
CIE whiteness index (108)
Brightness
1 2 3 4 5
Naked TiO2 1/75 1/25 1/15 1/7.5
75.74 76.54 77.73 78.27 80.20
96.12 96.23 96.39 96.62 96.97
Red-green index 0.09 0.07 0.06 0.08 0.05
Yellow-blue index
Relative light scattering ratio
3.24 3.13 2.95 2.96 2.73
100 107 108 102 87
The samples were prepared at a pH value of 9.5 and a reaction temperature of 90 8C with different mole ratios of Na2SiO3 to TiO2.
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the light scattering property of rutile TiO2 is better than that of SiO2.
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measurement of the samples. This work was financially supported by research funds from Chinese Education Department (2003406) and Jiangsu University (1281310001).
4. Conclusions The morphology of the SiO2 coating layers on the surface of TiO2 powders can be controlled by adjusting the reaction temperature, pH value of the reaction solution, and the SiO2 loading. The island-like SiO2 coating layers can be formed on TiO2 surface when the reaction temperature, the pH value, and the mole ratio of Na2SiO3 to TiO2 are low. The continuous and uniform SiO2 coating layers with an average thickness of 11 nm are formed in a pH range from 9 to 10 when the mole ratio of Na2SiO3 to TiO2 is 1:7.5. The thickness of the coating layer is increased with the increase of the mole ratio of Na2SiO3 to TiO2. The SiO2 coating layers are anchored on the TiO2 surface by the Ti–O–Si bonding. The dispersibility of the SiO2-coated TiO2 powders is affected by the morphology of the SiO2 coating. The whiteness and brightness of the SiO2-coated TiO2 powders increase with the increase of the SiO2 loading while the relative light scattering index has a maximum value. Acknowledgements The authors thank Prof. Weidong Zhou (Analysis Center, Yangzhou University) very much for kindly supporting TEM
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Applied Surface Science 254 (2008) 2809–2819 www.elsevier.com/locate/apsusc
Effects of coating parameters on the morphology of SiO2-coated TiO2 and the pigmentary properties Yumin Liu, Chen Ge, Min Ren, Hengbo Yin *, Aili Wang, Dongzhi Zhang, Chunyan Liu, Jun Chen, Hui Feng, Hengping Yao, Tingshun Jiang Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China Received 4 October 2007; received in revised form 11 October 2007; accepted 11 October 2007 Available online 18 October 2007
Abstract SiO2-coated TiO2 powders were prepared by the chemical deposition method starting from rutile TiO2 and Na2SiO3. The SiO2-coated TiO2 powders were characterized by X-ray photoelectron spectroscopy, Zeta-potential analysis, Fourier transform infrared spectroscopy, and transmission electron microscopy. The evolution of island-like and uniform coating layers was found to depend upon the ratio of Na2SiO3 to TiO2, reaction temperature, and pH. The whiteness and brightness of the SiO2-coated TiO2 powders increased in response to an increase in the SiO2 loading, but there was a maximum value among the light scattering indexes. The SiO2-coated TiO2 powders possessed more negative Zeta potentials than the naked TiO2. The dispersibility of the SiO2-coated TiO2 powders with the continuous and uniform SiO2 coating layers was higher than that of the naked TiO2 and the SiO2-coated TiO2 powders with the island-like SiO2 coating layers. # 2007 Elsevier B.V. All rights reserved. Keywords: Rutile TiO2; SiO2; Island-like coating; Uniform coating; Pigmentary property
1. Introduction Commercial TiO2 powders with a particle size of more than 100 nm are widely used as a white pigment in paints, plastics, and the paper industry on account of its nontoxicity, its opacity, its high chemical stability, and its excellent whiteness and brightness. Since TiO2 has a strong photocatalytic activity, the covering of the TiO2 surface with nonactive oxides, such as SiO2, Al2O3, and ZrO2, is important in suppressing its photoactivity and improving its weather durability [1–3]. Rutile TiO2 powders are industrially produced by two methods: chloride and sulfate. The chloride method is via the gas phase reaction of TiCl4 with O2 at 1700 8C to produce TiO2 powders directly. The sulfate method is via the calcination (at ca. 1000 8C) of the precipitates produced by hydrolysis of titanyl sulfate. Rutile TiO2 powders produced by either method must then be coated with protective layers of SiO2 and A12O3 through wet chemistry processes in order to decrease their
* Corresponding author. Tel.: +86 511 88787591. E-mail address: [email protected] (H. Yin). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.10.021
photoactivity, to increase their weather durability, and to increase their dispersibility in liquids [1–3]. The rutile TiO2 powders produced by the chloride method have a narrower particle size distribution and a smoother surface than that produced by the sulfate method. After covering the inert layer on the TiO2 surface, the former has better pigmentary properties than the latter. Although several patents dealt with the covering of the rutile TiO2 powders in order to improve its pigmentary properties and its weather durability [4–6], few papers discuss in great detail the effects of the experimental conditions on the coating process of the inert layers and the effected optical properties [3]. In our present study, we investigated the effects of such coating parameters as reaction temperature, pH, and SiO2 loading. We examined the morphology of the SiO2 layer that forms on the surface of rutile TiO2 that was produced by the sulfate method. The experimental results show that both island-like and uniform coating layers can be manipulatively achieved by changing these experimental parameters. The coating was observed by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy.
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2. Experimental 2.1. Materials Rutile TiO2 powders with an average particle size of 300 nm were supplied by Zhenjiang Taibai Company as a gift. The rutile TiO2 powders were produced by the sulfate method without coating. The other chemicals, such as Na2SiO3, sulfuric acid, and sodium hydroxide, were of reagent grade and were used without further purification. Distilled water was used throughout all of the experiments.
for 20 min prior to the measurement. Zeta-potential measurement of the samples was conducted on a Zeta-potential meter (BDL-B). 0.2 g of the samples was dispersed in 200 ml of distilled water and the suspension was ultrasonicated for 20 min prior to Zeta-potential measurement. The brightness, whiteness, and light scattering index of the samples were analyzed using a spectrophotometer (CM-2500d), Minolta Co. Ltd., XL-30, and D65 illuminant. 3. Results and discussion 3.1. Effect of reaction temperature on the covering extent
2.2. Sample preparation Seventy-five grams of rutile TiO2 powder were ultrasonically treated in 750 ml of water for 20 min to obtain a welldispersed TiO2 suspension. The TiO2 suspension was transferred into a 2000 ml flask, kept in a water bath, and heated to a prescribed temperature. The TiO2 suspension was adjusted to a specific pH value by adding a NaOH (0.5 mol/l) aqueous solution. Aqueous solutions of Na2SiO3 (0.5 mol/l) and H2SO4 (0.1 mol/l) were added to the TiO2 suspension with two constant flow pumps. The flow rate of the Na2SiO3 aqueous solution was kept constant and the feeding time was fixed at 3 h. The pH value of the reaction solution was kept constant during the coating process by adjusting the flow rate of the sulfuric acid. After feeding, the resultant suspension was aged for 2 h at the specific pH. The precipitate was filtrated and washed with distilled water until the conductivity of the filtrate was less than 20 mS/m. Then the washed precipitate was dried in an electric oven at 105 8C for 2 h. The dried samples were kept in a desiccator for characterization. 2.3. Characterization The as-prepared samples were observed by transmission electron microscopy (TEM, Philips Tecnnai-12, operating at 120 kV) to investigate the effects of the coating parameters on the morphology and the thickness of the coating layer of the SiO2coated TiO2 powders. The samples for TEM were prepared by dispersing a small amount of the SiO2-coated TiO2 powders in ethanol and ultrasonicated for 10 min. Then a few drops of the resultant suspension were dropped onto a copper grid coated with a layer of amorphous carbon. The Fourier transform infrared spectra of the SiO2-coated TiO2 powders were performed by the KBr pellet technique on a Fourier transform infrared spectrometer (Nicolet Nexus470) to determine the interfacial chemical bonding structure. XPS analysis was carried out on a VG ESCALB spectrometer equipped with Mg Ka X-ray source, operating at 300 W. For all the samples, the spectra of C 1s, Si 2p, O 1s, and Ti 2p were recorded. The binding energies were referred to the C 1s binding energy at 284.5 eV. The average particle size and the size distribution of the samples were determined by a particle size distribution analyzer (BIC-90 Zeta pals, Brookhaven Instruments Inc.) with Dynamic Laser Scattering (DLS) mode. 0.1 g of the samples was dispersed in 150 ml of distilled water and the suspension was ultrasonicated
Fig. 1 shows the TEM images of the naked TiO2 and the SiO2-coated TiO2 powders prepared at different reaction temperatures. The pH value of the reaction solution was 9.5 and the mole ratio of Na2SiO3 to TiO2 was 1:25. As certified by the TEM image, the TiO2 surface was not coated by SiO2 when the reaction temperature was 30 8C. When the reaction temperature was raised to 50 8C, the loosely islandlike SiO2 coating layer formed on the TiO2 surface. The average particle size of the SiO2 particles (the islands), parallel to TiO2 surface, was ca. 7.5 nm and the average thickness of the SiO2 coating layers was ca. 4 nm. Raising the reaction temperature to 80 8C caused a denser island-like SiO2 coating layer to form. The average particle size of the SiO2 particles increased to approximately 9.5 nm and the average thickness of the islandlike SiO2 coating layers increased to ca. 5 nm. Further raising the reaction temperature to 90 8C increased the average particle size of the denser island-like SiO2 coating layer to 13 nm. The densely continuous and uniform SiO2 coating layers formed with an average thickness of 5 nm. Therefore, it can be concluded that raising the temperature of this reaction causes the formation of a dense SiO2 coating layer on the surface of the rutile TiO2 powder. 3.2. Effects of pH and Na2SiO3 content on the morphology of SiO2 coating layer To investigate the effects of the pH value of the reaction solution and the Na2SiO3 content on the morphology of the SiO2 coating layer on the TiO2 surface, the SiO2-coated TiO2 samples were prepared at a given reaction temperature of 90 8C, the mole ratios of Na2SiO3 to TiO2 were 1:25, 1:15, and 1:7.5, respectively, and the pH values of the reaction solutions were changed from 7 to 11. Fig. 2 shows the TEM images of the SiO2-coated TiO2 samples prepared with a given mole ratio of Na2SiO3 to TiO2 of 1:25 at the different pH values of the reaction solutions. From the TEM images, it can be found that the naked TiO2 powders have smooth surfaces and the average primary particle size is ca. 300 nm. When the covering process was carried out at the pH values ranging from 7 to 9, the SiO2 coating layers were found to be composed of SiO2 particles with an average particle size of ca. 15 nm. The average thickness of the resultant islandlike coating layers was ca. 8 nm. When the pH value was increased to 9.5, the surface of the TiO2 was partially covered
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Fig. 1. TEM images of the SiO2-coated TiO2 samples prepared at a pH value of 9.5 and different reaction temperatures with a mole ratio of Na2SiO3 to TiO2 of 1:25.
by the densely continuous and uniform coating layers with an average thickness of 5 nm. Also, the island-like coating layers formed with an average particle size of ca. 13 nm. When the pH value was further increased to 10, only a trace of the densely uniform coating layer was found on the TiO2 surface. When the pH value was more than 10, the surface of the TiO2 powders was the same as that of the naked TiO2 powders. There was no SiO2 coating layer found on the TiO2 powder surface. The TEM images of the SiO2-coated TiO2 samples prepared at the different pH values are shown in Fig. 3 for a mole ratio of Na2SiO3 to TiO2 of 1:15. As the pH values ranged from 7 to 8.5, the densely island-like SiO2 coating layers formed. The average particle size of the SiO2 particles was ca. 18 nm and the thickness of the densely island-like coating layers was ca. 8 nm. When the pH values were increased to 9 and 10, the densely continuous and uniform SiO2 coating layers were dominantly formed and the average thicknesses of the SiO2 coating layers were 8 and 5 nm, respectively. When the pH value of the
reaction solution was more than 10, a SiO2 coating layer was not formed on the TiO2 surface. As the mole ratio of Na2SiO3 to TiO2 was increased to 1:7.5, TEM images show that the densely island-like SiO2 coating layers were dominantly formed when the pH value of the reaction solution was 7 (Fig. 4). The average particle size of the SiO2 particles was 20 nm and the average thickness of the SiO2 coating layers was 13 nm. While increasing the pH values from 7 to 8, 8.5 and 9, the morphologies of the SiO2 coating layers changed from densely island-like coating to densely continuous and uniform coating. When the pH values were 9–10, the densely continuous and uniform coating layers were dominantly formed with an average thickness of ca. 11 nm. When the pH value reached 10.5, the covering extent was low. There was no SiO2 coating layer found when the pH value was more than 10.5. The results show that the thickness of the SiO2 coating layer was increased by increasing the mole ratio of Na2SiO3 to TiO2
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Fig. 2. TEM images of the SiO2-coated TiO2 samples prepared at a reaction temperature of 90 8C and different pH values with a mole ratio of Na2SiO3 to TiO2 of 1:25.
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Fig. 3. TEM images of the SiO2-coated TiO2 samples prepared at a reaction temperature of 90 8C and different pH values with a mole ratio of Na2SiO3 to TiO2 of 1:15.
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Fig. 4. TEM images of the SiO2-coated TiO2 samples prepared at a reaction temperature of 90 8C and different pH values with a mole ratio of Na2SiO3 to TiO2 of 1:7.5.
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Fig. 4. (Continued ).
Fig. 5. FTIR spectra of the SiO2-coated TiO2 samples prepared at a pH value of 9.5 and a reaction temperature of 90 8C with different mole ratios of Na2SiO3 to TiO2: (a) naked TiO2; (b)–(e) the mole ratios of Na2SiO3 to TiO2 were 1:75, 1:25, 1:15, and 1:7.5, respectively.
Fig. 6. X-ray photoelectron spectra of O 1s of the samples: (a) naked TiO2 and (b)–(d) samples prepared at a reaction temperature of 90 8C and a pH value of 9.5 with different mole ratios of Na2SiO3 to TiO2 (b, 1:75; c, 1:25; d, 1:7.5).
Fig. 7. X-ray photoelectron spectra of Ti 2p of the samples: (a) naked TiO2 and (b)–(d) samples prepared at a reaction temperature of 90 8C and a pH value of 9.5 with different mole ratios of Na2SiO3 to TiO2 (b, 1:75; c, 1:25; d, 1:7.5).
Fig. 8. X-ray photoelectron spectra of Si 2p of the samples prepared at a reaction temperature of 90 8C and a pH value of 9.5 with different mole ratios of Na2SiO3 to TiO2 (b, 1:75; c, 1:25; d, 1:7.5).
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and that the densely continuous and uniform SiO2 coating layers formed in a large range of pH values when the mole ratio of Na2SiO3 to TiO2 was high. On the other hand, when the pH value of the reaction solution was more than 10.5, SiO2 coating layers were not formed on the surface of the TiO2 powders. 3.3. IR analysis Fig. 5 shows the FT-IR spectra of the naked TiO2 powders and the SiO2-coated TiO2 samples prepared at a pH value of 9.5 and a reaction temperature of 90 8C with the different mole ratios of Na2SiO3 to TiO2. The bands at 800–400 cm 1 assigned to Ti–O–Ti vibrations were observed over all of the samples. The band at ca. 955.73 cm 1 occurred only in the spectra of the SiO2-coated TiO2 samples, indicating that SiO2 coating layers were anchored on the TiO2 surface by the formation of Ti–O–Si bonds [7,8]. While increasing the mole ratio of Na2SiO3 to TiO2, the intensity of the band at 955.73 cm 1 decreased due to the thicker SiO2 coating layer. Furthermore, the bands at 1089.24 and 1231.39 cm 1 assigned to the asymmetric stretching vibration modes of the Si–O–Si bridge [8,9] were observed when the mole ratio of Na2SiO3 to TiO2 was 1:25. The band intensities increased when the mole ratio of Na2SiO3 to TiO2 increased, revealing that the thickness of the SiO2 coating layer increased along with the SiO2 loading. 3.4. XPS analysis Fig. 6 shows the O 1s peaks of the naked rutile TiO2 and SiO2-coated TiO2 samples. The binding energy of O 1s peak of the naked rutile TiO2 was 529.1 eV. When the SiO2-coated TiO2 samples were prepared with the mole ratios of Na2SiO3 to TiO2 of 1:75, 1:25, and 1:7.5, the binding energies of O 1s peaks were 529.4, 532.25, and 532.65 eV, respectively. The O 1s peak located at around 529 eV should be ascribed to that of rutile TiO2 and the O 1s peak at around 532 eV ascribed to that of SiO2. The shift of the O 1s peaks certifies that SiO2 reacted on the TiO2 surface through the Ti–O–Si chemical bond at the interface of the SiO2 coating layer and the TiO2 particle, which is consistent with the IR analysis. The binding energy of Ti 2p3/2 of the naked rutile TiO2 was 457.65 eV (Fig. 7). When the SiO2-coated TiO2 samples were prepared with the mole ratios of Na2SiO3 to TiO2 of 1:75, 1:25, and 1:7.5, the binding energies of the Ti 2p3/2 and Si 2p of the samples were 457.95, 101.65; 458.3, 103; and 458.3, 103.45 eV, respectively (Figs. 7 and 8). While increasing SiO2 loading, the shifts of the Ti 2p3/2 and Si 2p peaks toward high binding energy reveal that the chemical states of Si and Ti changed. When the mole ratios of Na2SiO3 to TiO2 were increased to 1:25 and 1:7.5, the binding energies of the Ti 2p3/2 for both samples remained the same, indicating that the chemical state of Ti atoms at the interface was not affected by further increasing the SiO2 loading, i.e. the reaction between TiO2 surface and the SiO2 coating layer was complete. This result also indicates that the TiO2 surface was completely covered by SiO2 coating layers when the mole ratio of Na2SiO3 to TiO2 was increased to 1:25. On the other hand, the binding
energies of Si 2p ceaselessly increased and reached that of bulk SiO2 (103.35 eV) [10] with the increasing of the SiO2 loading. From the TEM analysis, it was found that the SiO2 coating layer existed in both densely island-like and uniform coating styles on the TiO2 surface when the mole ratio of Na2SiO3 to TiO2 was 1:25 and that the SiO2 coating layer dominantly existed in the uniform coating style when the mole ratio of Na2SiO3 to TiO2 was increased to 1:7.5. Therefore, it is reasonable to conclude that the morphology of the SiO2 coating layer affects the chemical state of Si atom in addition to the chemical bond between the Si of a SiO2 coating layer and the Ti of TiO2 via O atom at their interface. On the other hand, other researchers reported that SiO2 coating on TiO2 was still amorphous even when calcined at a high temperature of up to 1100 8C [11]. Therefore, the SiO2 coating layer formed under our present experimental conditions should exist in amorphous phase. 3.5. Coating process of SiO2 layer The isoelectric point (IEP) of rutile TiO2 is around pH 3.5 [3,12]. Under the present experimental conditions, the pH value of the reaction solution was greater or equal to 7; the rutile TiO2 surface was negatively charged. When the pH values of the reaction solutions were in a range of 7–8, Na2SiO3 was rapidly hydrolyzed to form a large number of siliceous micelles. The resultant siliceous micelles were anchored on the surface of TiO2 powders via Ti–O–Si bonding to obtain the island-like coating layers. Increasing the pH value to 9–10 should lower the hydrolysis rate of Na2SiO3 resulting in the formation of small-sized or less aggregated siliceous micelles. First, the small-sized micelles were anchored on the surface of TiO2 powders via Ti–O–Si bonding. Then both the micelles present in the solution and the anchored micelles condensed via Si–O–Si bonding to obtain continuous and uniform coating layers. When the pH value was more than 10.5, silicon species exist as single silicate anions, or less aggregated
Fig. 9. Zeta potentials of the SiO2-coated TiO2 samples prepared at a pH value of 9.5 and a reaction temperature of 90 8C with different mole ratios of Na2SiO3 to TiO2.
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siliceous micelles with quite small particle size, which should be more negatively charged. The single silicate anions and the highly negatively charged siliceous micelles did not react with the negatively charged TiO2 surface due to the strong electrostatic repulsion. Therefore, no SiO2 coating layers were formed on the TiO2 surface at this higher pH value. 3.6. Zeta-potential measurement The Zeta potentials of the naked TiO2 and the SiO2-coated TiO2 samples are shown in Fig. 9. After coating SiO2, the Zeta
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potentials rapidly decreased from 24 mVof the naked TiO2 to ca. 50 mVof the SiO2-coated TiO2 samples, revealing that the SiO2-coated TiO2 samples were more negatively charged than the naked TiO2. The Zeta potentials of the SiO2-coated TiO2 samples roughly kept constant while varying the mole ratio of Na2SiO3 to TiO2 from 1:75 to 1:7.5. The results indicate that the charging property of the TiO2 surface was significantly affected by the SiO2 coating rather than the thickness of the SiO2 coating layers. Furthermore, the more negatively charged SiO2-coated TiO2 powders with higher electrostatic repulsion will result in finer particle dispersibility.
Fig. 10. Particle size distributions of the samples prepared at a reaction temperature of 90 8C and different pH values with a mole ratio of Na2SiO3 to TiO2 of 1:15.
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Fig. 11. Particle size distributions of the samples prepared at a reaction temperature of 90 8C and a pH value of 9.5 with different mole ratios of Na2SiO3 to TiO2.
3.7. Effects of pH and Na2SiO3 content on the dispersibility of SiO2-coated TiO2 Fig. 10 shows that the particle size distribution of the naked TiO2 powders was classified into two categories: the primary particles had an average particle size of 250 nm with a particle number percentage of 87% and the secondary particles had an average particle size of 800 nm with a particle number percentage of 13%. For the SiO2-coated TiO2 samples prepared with a mole ratio of Na2SiO3 to TiO2 of 1:15 at pH values of 7, 8.5, 9, 9.5, and 10, the particle size distributions were also classified into two categories (Fig. 10): the average particle sizes and the particle number percentages of the primary particles were 250 nm, 75%; 250 nm, 76%; 300 nm, 98%; 300 nm, 100%; and 300 nm, 95%, respectively, and the average particle sizes and the particle number percentages of the secondary particles were 800 nm, 25%; 800 nm, 24%; 800 nm, 2%; 800 nm, 0%; and 900 nm, 5%. When the samples were prepared at the high pH values between 9 and 10, the dispersibilities of the resultant samples were higher than that of the naked TiO2. As certified by TEM analysis, the island-like coating layers formed at the pH values between 7 and 8.5, and the continuous and uniform coating layers formed at the pH values between 9 and 10. Therefore, it is reasonable to conclude that the formation of the continuous and uniform coating is beneficial to the dispersion of the SiO2-coated TiO2 powders in water. To investigate the effect of the SiO2 loading on the dispersibility of the SiO2-coated TiO2 powders in water, the
particle size distributions of the samples prepared at a pH value of 9.5 with the mole ratios of Na2SiO3 to TiO2 of 1:25 and 1:7.5 were also determined by DLS technique, respectively (Fig. 11). The average particle sizes and the particle number percentages of the primary particles were 300 nm, 96%; and 300 nm, 94%, respectively, and the average particle sizes and the particle number percentages of the secondary particles were 1000 nm, 4%; and 900 nm, 6%. Comparing the dispersibilities of the naked TiO2 and the samples prepared with the mole ratios of Na2SiO3 to TiO2 of 1:25, 1:15, and 1:7.5, we found that the well-dispersed SiO2-coated TiO2 powders were obtained when the mole ratio of Na2SiO3 to TiO2 was 1:15. 3.8. The pigmentary property of the SiO2-coated TiO2 sample According to Table 1, it was found that the whiteness and brightness of the SiO2-coated TiO2 samples increased in response to an increase in the SiO2 loading. This is because of the colored metal impurity, such as with Fe3+ ions, present in the naked TiO2. The surface was further hidden with the increase of the SiO2 loading. On the other hand, the relative light scattering index had a maximum value with the increase of the SiO2 loading. It is commonly believed that the relative light scattering index is affected by the dispersibility and the intrinsic properties of the particles. The presence of the maximum relative light scattering index has two explanations: (1) there was an optimum value for SiO2 loading to obtain the well-dispersed SiO2-coated TiO2 powders and (2)
Table 1 Color schemes of SiO2-coated TiO2 samples (in system CIE) Samples
Mole ratios of Na2SiO3 to TiO2
CIE whiteness index (108)
Brightness
1 2 3 4 5
Naked TiO2 1/75 1/25 1/15 1/7.5
75.74 76.54 77.73 78.27 80.20
96.12 96.23 96.39 96.62 96.97
Red-green index 0.09 0.07 0.06 0.08 0.05
Yellow-blue index
Relative light scattering ratio
3.24 3.13 2.95 2.96 2.73
100 107 108 102 87
The samples were prepared at a pH value of 9.5 and a reaction temperature of 90 8C with different mole ratios of Na2SiO3 to TiO2.
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the light scattering property of rutile TiO2 is better than that of SiO2.
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measurement of the samples. This work was financially supported by research funds from Chinese Education Department (2003406) and Jiangsu University (1281310001).
4. Conclusions The morphology of the SiO2 coating layers on the surface of TiO2 powders can be controlled by adjusting the reaction temperature, pH value of the reaction solution, and the SiO2 loading. The island-like SiO2 coating layers can be formed on TiO2 surface when the reaction temperature, the pH value, and the mole ratio of Na2SiO3 to TiO2 are low. The continuous and uniform SiO2 coating layers with an average thickness of 11 nm are formed in a pH range from 9 to 10 when the mole ratio of Na2SiO3 to TiO2 is 1:7.5. The thickness of the coating layer is increased with the increase of the mole ratio of Na2SiO3 to TiO2. The SiO2 coating layers are anchored on the TiO2 surface by the Ti–O–Si bonding. The dispersibility of the SiO2-coated TiO2 powders is affected by the morphology of the SiO2 coating. The whiteness and brightness of the SiO2-coated TiO2 powders increase with the increase of the SiO2 loading while the relative light scattering index has a maximum value. Acknowledgements The authors thank Prof. Weidong Zhou (Analysis Center, Yangzhou University) very much for kindly supporting TEM
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