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Dispersibility and Hydrophobicity Analysis of Titanium Dioxide Nanoparticles Grafted with Silane Coupling Agent Chaoxia Wang,* Haiyan Mao, Chunxia Wang, and Shaohai Fu Key Laboratory of Eco-Textile, Ministry of Education, Jiangnan University, 1800 Lihu Road, Wuxi, 214122, People’s Republic of China
bS Supporting Information ABSTRACT: The high surface energy of TiO2 nanoparticulate leads to the aggregation of nanoparticles. The grafted modification of nanoparticles is necessary in order to improve its dispersibility in the organic solvent. The effect of grafted modification with silane coupling agent on dispersibility and lipophilicity of TiO2 nanoparticles was investigated. The factors of the modification were discussed, such as the amount of silane coupling agent, pH value, and reaction time. The average sizes of the TiO2 nanoparticles were 200300 nm. The thermogravimetric analysis (TGA) curves indicated that the TiO2 nanoparticle surface had been grafted with the silane coupling agent. The zeta potential showed that modified TiO2 could disperse well in the organic solvent. The lipophilic degree and contact angle suggested that the modified TiO2 had low surface energy and changed from a hydrophilic nature to a lipophilic nature.
’ INTRODUCTION Ink jet technology has been used successfully for printing textiles, aside from the printing of carpets and banners.1 Ink jet printing on textiles can give fine and delicate images on fabrics using a high-resolution device and printing system. Although there are specialized ink jet printing machines for textiles, printing on a textile is a huge project. Ink jet printing systems use low-viscosity inks to attain high jetting frequency from small nozzles.24 People also have more and more demands, reagrding the color of the printing patterns. BASF established inks based on all four colorants (cyan, magenta, yellow, and black colors).5 The use of smaller pigment particles in ink jet technology can improve not only the stability of the pigment in the dispersion medium, but also color strength, contrast, and transmittance.6 Also, traditional inorganic pigments, including titanium dioxide, zinc oxide, and silicon oxide, are available in a nanosize range for application in inks. However, the greatest problem of using nanopigments is the fact that the finer the powder, the higher the surface area, implying poor mixing and aggravation of agglomeration phenomena.7 With the increasing need for white ink printing for the dark fabrics, people have attached more and more importance to the white ink. Nanoparticle titanium dioxide (TiO2, rutile type) is the most widely used white pigment, because of its brightness, very high refraction index (n = 2.7), and high chemical stability.8 Especially, TiO2 nanoparticles have been applied as electrophoretic particles in the applications of electronic ink, which are novel image displays with low power consumption, high contrast ratio, wide angle view, etc.9 However, TiO2 in white ink has high density to deposit easily and stabilize uncomfortably. TiO2 does not dissolve in water, so it will precipitate slowly after a certain period of time. Many researchers have improved the stability of TiO2 particles by modification. The feasibility of using organic functional groups to produce chemical bonds on the particle surface was verified.1014 Xerox used a thermoplastic resin that hardened after it had precipitated on the pigment to improve the r 2011 American Chemical Society
chemical stability.15 A silane coupling agent and a titanate coupling agent were used for modification of inorganic particulates.16,17 Some researchers had reported that surface modification with silane coupling agents provided a thin layer on the inorganic oxide surface.18,19 However, the modification of inorganic oxide particles was usually carried out in an organic solvent, which created environmental problems. In this study, grafted modification of TiO2 nanoparticles with silane coupling agents was performed with ultrasound in the water system. The effects of silane coupling agents on dispersibility and surface properties of TiO2 nanoparticles were investigated. It is expected to provide some references for colorchangeable fabrics, which are based on E-ink display technology.
’ EXPERIMENTAL SECTION Materials. The hydrochloric acid and ammonia used in this paper were supplied by Sinopharm Chemical Reagent Co., Ltd. (China). The silane coupling agent (γ-methacrylic acyloxy propyl trimethoxysilane) and titanium dioxide (TiO2) were offered by Qufu Wanda Chemical Engineering Co., Ltd. and Shanghai Liangjiang Titanium Chemical.Co., Ltd. (China), respectively. The molecular structure of γ-methacrylic acyloxy propyl trimethoxysilane is shown in Figure 1. Grafted Modification of TiO2 Nanoparticles with a Silane Coupling Agent. Grafted modification of TiO2 nanoparticles was carried out in the liquid phase. The silane coupling agent was added in deionized water and then was combined with TiO2. The pH value of the slurry was adjusted to 310 via the addition of 0.1 mol/L NH3 3 H2O solution and 0.1 mol/L HCl solution. The slurry was subjected to ultrasonic treatment for a specified time. Received: April 24, 2011 Accepted: September 27, 2011 Revised: July 13, 2011 Published: September 27, 2011 11930
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Figure 1. The chemical structure of γ-methacrylic acyloxy propyl trimethoxysilane.
Figure 3. Effect of pH on the particle size.
Figure 2. Effect of modifier dosage on the particle size.
The reaction mixture was centrifuged (4000 rpm, 20 min). The modified powders were washed three times with deionized water and then dried overnight in a vacuum oven. Particle Sizes and Zeta Potentials of the Modified TiO2 Nanoparticles. Samples were prepared by mixing TiO2 powders (0.02 g) and EtOH (200 mL), which were subjected to ultrasonic treatment for 10 min. The zeta potentials and particle sizes of the samples were measured with a zeta potential measuring system and particle size analysis equipment (Nano-ZS90, Malvern Co., U.K.), respectively. Lipophilicity of the Modified TiO2 Nanoparticles. The grafted modified TiO2 powders (0.5 g) were added into deionized water (50 mL), using methanol titration. When the powders floating on the water surface were completely wetted, the volume of methanol was recorded.20 The lipophilicity of modified TiO2 particles was calculated using eq 1: LD ð%Þ ¼
V 100 V þ 50
ð1Þ
where LD is the lipophilic degree and V is the volume of methanol. Contact Angle of the TiO2 Nanoparticles. Bare TiO2 nanoparticles or modified TiO2 nanoparticles were dispersed in toluene, which were filmed in the slide and dried. A quantity of 0.6 μL of water was deposited dropwise on the slide at room temperature. The contact angle was measured via drop shape analysis for DSA100 (DSA1 v 1.9, Kruss Co., Germany). FTIR Analysis of the TiO2 Nanoparticles. Fourier transform infrared (FTIR) analysis of the bare TiO2 nanoparticles or modified TiO2 nanoparticles was performed using a Nexus 470 spectrometer (Thermo Nicolet, Madison, WI). The samples were prepared in KBr pellets. TGA Analysis of the TiO2 Nanoparticles. Bare TiO2 nanoparticles or modified TiO2 nanoparticles were determined by thermogravimetric analysis (TGA), using a Model TGA/SDTA851e thermogravimetric analyzer (Mettler Co., Switzerland). Samples were heated from room temperature to 800 °C at a rate of 10 °C min1.
Figure 4. Effect of reaction time on the particle size.
’ RESULTS AND DISCUSSION Particle Size of the TiO2 Nanoparticles. The refractive index of most resins in pigment is between 1.45 and 1.60. While the light scattering power is at its maximum, the relationship between the pigment particle size (D), the wavelength of incident rays (λ), the refractive index of pigment (n1), and the refractive index of the resin (n2) are expressed as follows in eq 2: D ¼ 2λ πðn1 n2 Þ ð2Þ
The wavelength range of visible light is 400700 nm. For TiO2 particles in the wavelength range of visible light, the most appropriate particle size range is usually 0.150.35 μm. Therefore, the particle size of TiO2 should be controlled at 0.15 0.35 μm, to achieve maximum scattering force and good whiteness. The subsequent particle size results showed that TiO2 grafted with a silane coupling agent still had good scattering force and whiteness. Figure 2 showed the particle size changes of TiO2 nanoparticles under the different modifier dosage. The methoxy groups of the silane coupling agent hydrolyzed and then were chemically bound up with TiO2 nanoparticles. When a smaller amount of the silane coupling agent was used, the size of the TiO2 nanoparticles increased. This could be due to the fact that TiO2 nanoparticles were not modified completely. The particle size of TiO2 nanoparticles with an initial silane coupling agent/ TiO2 mass ratio of 3:10 was 266.4 nm, which indicated that the TiO2 nanoparticles were coated completely with modifiers to disperse well in the organic solvent. Dynamic laser scattering (DLS) measurements have shown that the modified TiO2 particle size was dependent on the pH of the slurry. The pH of the suspension was adjusted via additions of 0.1 mol/L NH3 3 H2O solution and 0.1 mol/L HCl solution. The size distribution curves for the cases with different pH values of the slurry are represented in Figure 3. At pH 8 (Figure 3a), the size distribution was rather narrow with an average aggregate size 11931
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Figure 5. Effect of modifier dosage on the zeta potential.
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Figure 7. Effect of modifier dosage on the lipophilic degree (LD).
Figure 8. Contact angles of water droplets on the surface of unmodified TiO2 and modified TiO2.
Figure 6. Effect of pH on the zeta potential.
of 259.4 nm. If the pH value was 6 (Figure 3b), the particle size distribution was wider and shifted to the bigger size value. The average size of such TiO2 nanoparticles was 378.5 nm. It demonstrated that TiO2 was modified well with a silane coupling agent in the weak base. This could indicate that the methoxy groups of the silane coupling agent were well-hydrolyzed to modify TiO2 under the weak base. The multilayer adsorption predominated the adsorption of the silane coupling agent on TiO2 in the acid and TiO2 nanoparticles were aggregated to enlarge the particle size. The particle sizes of TiO2 at different reaction times were studied. There was a decrease in particle size with increased reaction time. The particle size was 338.4 nm at 15 min (Figure 4b), and after 30 min of reaction time, the particle size was 259.4 nm (Figure 4a). The TiO2 surface adsorption and the chemical bonding of the silane coupling agent and TiOH resulted in simultaneous changes in the particle size. As the reaction time increased, the amount of hydroxide radical on the TiO2 surface was reduced. At the same time, the hydroxide radical on the TiO2 surface could not be completely reacted, because of steric effects. Therefore, TiO2 nanoparticles after a reaction time of 15 min gathered easily and their particle size increased. Zeta Potential of the TiO2 Nanoparticles. According to the DLVO theory (which is named after Derjaguin, Landau, Verwey, and Overbeek, the researchers who developed it in the 1940s), an important factor influencing the stability of aqueous dispersions is the surface potential of the particles. Figure 5 shows that the zeta potential increased gradually at a silane coupling agent/TiO2
mass ratio of 0.10.3. The zeta potential of the TiO2 particle varied from 24.9 mV at a silane coupling agent/TiO2 mass ratio of 0.1 to 36.9 mV at a silane coupling agent/TiO2 mass ratio of 0.3, which was in good agreement with its stability. The zeta potential decreased slightly as its dosage continued to increase. From these experiments, it could be inferred that TiO2 nanoparticles were not modified completely with a shortage of silane coupling agent. However, the role between excessive silane coupling agent and the dispersion media also had a bad influence on the modification of the silane coupling agent. Therefore, the silane coupling agent dosage should be properly selected. It is well-known that the pH of the aqueous medium has a substantial effect on the stability of aqueous dispersion of TiO2 powders. The zeta potential of suspensions was related to the electrostatic stability of TiO2 nanoparticles, which also determined whether or not the nanoparticles would tend to flocculate in the suspensions. As could be seen from Figure 6, the zeta potential was 13.6 mV at pH 5, in which the surface of nanoparticles were charged less. The interaction force between nanoparticles was strong to flocculate easily. The zeta potential decreased as the pH increased. The density of surface charge and the electrostatic repulsion among them were strong at pH 8, and the zeta potential was 37.3 mV. This could be an indication that the silane coupling agent modified TiO2 well under alkali conditions. Lipophilic Degree of the TiO2 Nanoparticles. The modifier dosage played a more important role in the lipophilic degree (LD) than pH. As could be seen from Figure 7, the LD increased with increasing modifier dosage. The size of LD showed the dispersibility of the nanoparticles in organic medium. The surface of primitive TiO2 nanoparticles had some hydroxide radicals, so they were hydrophilic and sank in deionized water. However, the modified TiO2 nanoparticles floated the surface of the deionized water. This indicated that the surface properties of TiO2 changed drastically after modification. The methoxy groups of the silane coupling agent hydrolyzed. Reactive silane triols were formed and condensed to form oligomers, which were adsorbed on the 11932
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Figure 9. Fourier transform infrared (FTIR) spectra of modified TiO2 nanoparticles and bare TiO2 nanoparticles.
surface and condensed with surface hydroxyl groups to form bond linkages, resulting in a silane coupling agent that is chemically bonded on the surface of the TiO2 nanoparticles. Contact Angle of the TiO2 Nanoparticles. The wetting of water on the solid is dependent on the relationship between the interfacial tensions (water/air, water/solid, and solid/air). The ratio between these interfacial tensions determines the contact angle (θ) between the water droplet on a given surface and the surface. The influence of grafted modification on the water contact angle of TiO2 is shown in Figures 8a and 8b. With regard to Figure 8a, it was observed that the water contact angle of primitive TiO2 was 55.75°. Figure 8b showed that the water contact angle of modified TiO2 was 114.34°. [Note that a contact angle of 0° means complete wetting, and a contact angle of 180° corresponds to complete nonwetting.] Hydrophobic surfaces with low wettability and contact angles of ∼90° e θ e 120° have been known for a long time. Generally, the higher the angle, the lower the surface energy. However, decreasing the contact angle should lead to enlarged values of the hydrophilic surfaces. From the above results, it was concluded that TiO2 nanoparticles grafted with a silane coupling agent had produced hydrophobic surfaces and low surface energy. Extraction Property of TiO2 Nanoparticles in the Absolute Ethanol. The modified TiO2 nanoparticles were Soxhlet-extracted by ethanol for 10 h. The samples then were dried at 120 °C for analysis of the LD. Before Soxhlet extraction, LD = 59.3%. However, the lipophilic degree of the extraction was LD = 53.2% and it also floated on the surface of the deionized water. If the silane coupling agent had weak combinative ability with TiO2, it would desorb and there was no difference from premodified TiO2. In the presence of strong combinative ability, the silane coupling agent on the surface of the modified particle was still not extracted by a long solvent extraction. The results indicated that the modified TiO2 had good chemical stability in the organic solvent. FTIR Analysis. The FTIR spectra of modified TiO2 nanoparticles and bare TiO2 nanoparticles are shown in Figure 9. From curve (b) in Figure 9, the band corresponding to the surface hydroxyl groups on the TiO2 surface was generally observed at ∼3600 cm13400 cm1, because of overlapping with the peak related to physically adsorbed water on the TiO2 surface. However, this peak on curve (a) in Figure 9 became sharper, because
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Figure 10. Thermogravimetric analysis (TGA) of bare TiO2 nanoparticles and modified TiO2 nanoparticles.
hydroxyl groups on the TiO2 surface could not be completely involved in response, because of a steric hindrance effect, which generated hydrogen bond to intermolecular association. In addition, the band at ∼1119.42 cm1, which corresponded to the SiOSi bond, was observed, which indicated the condensation reaction between silanol groups. The band corresponding to the TiOTi bond generally appeared at ∼800 cm1500 cm1. The band at 2924.66 cm1 was attributed to CH3 and CH2 stretching. CdO and CdC stretching occurred at 1719.65 cm1 and 1632.26 cm1, respectively. From the inset in Figure 9 (showing an enlarged view of the ∼1000880 cm1 range), the FTIR spectra of modified TiO2 nanoparticles assigned the peak at 970 cm1 to the stretch vibration band of TiOSi. According to the FTIR analysis, it could be concluded that the silane coupling agent was successfully grafted onto the surface of TiO2 nanoparticles. TGA Analysis. Figure 10 shows TGA curves of the bare TiO2 nanoparticles and modified TiO2 nanoparticles. As seen in TGA curve (a) in this figure, the decrease in weight of bare TiO2 nanoparticles was 2.1% when the temperature was
