Preparation and characterization of TiO 2 nanoparticles surface-modified by octadecyltrimethoxysilane

Indian Journal of Engineering & Materials Sciences Vol. 20, December 2013, pp. 561-567 Preparation and characterization of TiO 2 nanoparticles surface-modified by octadecyltrimethoxysilane Hui Pan a, XiaoDong Wang b, ShaSha Xiao b, LaiGui Yu b & ZhiJun Zhang b a College of Chemistry and Chemical Engineering, b Key Laboratory of Ministry of Education for Special Functional Materials, Henan University, Kaifeng 475004, China Received 9 April 2012; accepted 24 July 2013 Titanium dioxide (TiO 2 ) nanoparticles surface-modified by octadecyltrimethoxysilane (commercial name WD-11) are prepared via in situ surface-modification in dehydrated ethanol at room temperature. As-obtained TiO 2 nanoparticles are characterized by means of Fourier transform infrared spectrometry, thermogravimetric, X-ray diffraction and transmission electron microscope. The tribological properties of TiO 2 nanoparticles surface-capped by WD-11 as an additive in liquid paraffin are evaluated with a four-ball friction and wear tester. It is found that WD-11 is chemically bonded to the surface of TiO 2 nanoparticles during the surface modification process, resulting in increased dispersibility of TiO 2 nanoparticles in various organic solvents. UV-vis transmittance spectra show that the light transmittance of modified TiO 2 nanoparticles in liquid paraffin (1%, w/v) is over 99%, which indicates that highly dispersed TiO 2 nanoparticles can be obtained. Besides, surface-capped TiO 2 nanoparticles are able to significantly improve the antiwear ability of liquid paraffin, showing promising application as a multifunctional lubricating oil additive of steel-steel frictional pair. Keywords: Titanium dioxide nanoparticles, Surface-modification, Octadecyltrimethoxysilane, Dehydrated ethanol, Antiwear As one of the most promising nanomaterials, TiO 2 nanoparticles have aroused a great interest in catalysis, coating, chemical sensing, and many other scientific fields, due to peculiar properties advantageous over bulk TiO 2 or micro-sized TiO 2 particulates 1-4. Unfortunately, TiO 2 nanoparticles with a high ratio of surface area to volume tend to agglomerate, which severely impedes their application. To prevent inorganic nanoparticles with a high surface activity from agglomeration, researchers have employed various methods to stabilize nanoparticles, where Langmuir-Boldgett films, vesicles, and surface-active supports have been extensively focused on 5. Of various methods for stabilizing inorganic nanoparticles, surface chemical modification with organic molecules is of particular significance, because it can be utilized to effectively prevent agglomeration and increase the dispersibility and stability of nanoparticles in many organic solvents 6-12. Typically, silane coupling agents are used as modifiers and improve the surface properties of the obtained nanoparticles. Nakayama and Hayashi 13 prepared TiO 2 nanoparticles surface-modified by both carboxylic acid and amine (two-step method) *Corresponding author: (E-mail: panhui@henu.edu.cn) and found that the dispersibility and stabilization in ethanol or toluene are better than TiO 2 nanoparticles surface-modified by either acid or amine (one-step method). Jesionowski 14 reported the surface modification of titanium dioxide (rutile) by silane coupling agents and found that octylsilane had induced a most pronounced increase in hydrophobicity of titanium white 14. Wang et al. 15 reported that the modified TiO 2 with r-methacrylic acyloxy propyl trimethoxysilane had good dispersibility and chemical stability in the organic solvent, where they described TiO 2 nanoparticles grafted with the silane coupling agent had produced hydrophobic surfaces and low surface energy. Zhao et al. 16 also reported that the surface of commercial TiO 2 nanoparticles was modified by 3-aminopropyltrimethoxysilane and 3- isocyanatopropyltrimethoxysilane. Xue et al. 17 synthesized TiO 2 nanoparticles surface-modified by 2-ethyl hexoic acid and found that as-obtained surface-capped nano-sized TiO 2 as a lubricating oil additive was able to greatly improve the tribological properties of the base stock. After all, the TiO 2 nanoparticles modified by the shorter chain alkyl surfactants can be only dispersed into few kinds of organic solvents. To the best of our knowledge, however, there have

562 INDIAN J. ENG. MATER. SCI., DECEMBER 2013 been few reports on preparation of TiO 2 nanoparticles surface-modified by silane coupling reagent like octadecyltrimethoxysilane (commercial name WD-11) which possesses hydrophobic group of much quantity (in other words, using long alkyl chain) and better oil solubility. Therefore, in this study, TiO 2 nanoparticles were synthesized using a sol-gel method 18,19, followed by in situ surface-modification with silane coupling reagent WD-11 of a long carbon chain. In comparison with previous results, it is a very economical method, which is applicable to distribution of a variety of organic solvents without changing the properties of nanoparticles. Such an organo-modifier not only can prevent TiO 2 nanoparticles from aggregation, but also can endow inorganic nanoparticles with significantly improved dispersibility and stability in various organic solvents such as paraffin liquid, dibutylphthalate, toluene, and so on. As-obtained surface-capped TiO 2 nanoparticles were characterized by means of Fourier transform infrared spectrometry, thermogravimetric, X-ray diffraction and transmission electron microscopy. The tribological properties of as-synthesized surfacecapped nanoscale TiO 2 as an additive in liquid paraffin were evaluated with a four-ball friction and wear tester. Experimental Procedure Materials Silane coupling agent CH 3 (CH 2 ) 17 Si(OCH 3 ) 3 (commercial name WD-11; technical grade) was provided by Silicone New Material Company Ltd of Wuhan University (Wuhan, China). Analytical grade tetrabutyl titanate was provided by Tianjin Kermel Chemical Reagents Development Centre (Tianjin, China). Analytical grade acetic acid glacial, triethylamine and other reagents were all commercially obtained. Preparation of bare TiO 2 nanoparticles 10 ml of glacial acetic acid were dissolved in 120 ml of dehydrated ethanol, followed by slow addition of 30 ml of tetrabutyl titanate, then the obtained pale yellow solution was vigorously stirred at room temperature for 30 min, followed by slow addition of 3 ml of distilled water and 10 ml of triethylamine. The reaction solution was allowed to react at room temperature for 6 h under vigorous stirring. At the end of reaction, ethanol solvent was evaporated by distillation, the obtained sol was dried at 120ºC for 12 h and then rubbed to form powder. The powder was washed with distilled water for three times and extracted with soxhlet extractor for 12 h in dehydrated ethanol, finally the white nano-sized TiO 2 powder were obtained after the extracted samples were dried at 120ºC. Preparation of TiO 2 nanoparticles modified by WD-11 7.5 g of WD-11 and 10 ml of acetic acid glacial were dissolved in 120 ml of dehydrated ethanol, followed by slow addition of 30 ml of tetrabutyl titanate. Resultant mixed solution was vigorously stirred at room temperature for 30 min, followed by slow addition of 4 ml of distilled water and 10 ml of triethylamine. The mixed solution was then allowed to react at room temperature for 8 h under vigorous stirring. At the end of reaction, ethanol solvent was evaporated by distillation yielding a flaxen sol. The sol was dried at 80ºC for 48 h and then rubbed to form powder. As-obtained powder was alternately washed with ethanol and distilled water for three times, then the cake was extracted for 12 h to ensure that the most physically adsorbed WD-11 on the particle surface was removed. Final samples for characterization were obtained after the extracted cakes were dried at 80ºC for 24 h. Characterization and evaluation of tribological properties Fourier transformed infrared (FTIR) spectra were recorded on an Avatar360 FTIR spectrograph. The samples for FTIR were prepared using the KBr technology. KBr was used as the mulling agents. The dried TiO 2 nanoparticles and KBr were directly tableted as the sample to be measured. The spectra were taken in the range from 4000 to 500 cm -1. X-ray diffraction (XRD) spectra were determined on a X Pert Pro powder diffractometer (Philips, Holland, Cu Ka, λ= 0.154 nm). Thermogravimetric analyses were performed on a TGA/SDTA851e instrument (METTLER TOLEDO) from room temperature to 800ºC at a heating rate of 10ºC min -1 in nitrogen. Transmission electron microscopy (TEM) analyses were performed using a JEM-2010 (JEOL, Japan) operating at 200 kv. UV-vis absorption spectra of the as-prepared solution were recorded on a PE-Lambda35 spectrophotometer using quartz cell with a path length of 1 mm. The antiwear properties of the WD-11/TiO 2 nanoparticles as an additive in liquid paraffin were examined on a MRS-10A four-ball tester, in comparison with the base oil, the oil with the bare TiO 2 nanoparticles and WD-11. The additive

PAN et al.: TIO 2 NANOPARTICLES SURFACE MODIFIED BY OCTADECYLTRIMETHOXYSILANE 563 concentration of WD-11/TiO 2 in liquid paraffin were 0%, 0.1%, 0.3%, 0.5%, 1%, 2%, 4% and 6%, respectively. The concentrations of both bare TiO 2 and WD-11 in the base oil were 0.5%. The wear tests were conducted at a rotating speed of 1450 rpm and under a constant load of 300 N, for the test duration of 30 min. The balls (diameter in 12.7 mm) used in the tests were made of GCr15 steel with a HRC of 61-64. Before each test, the steel balls were cleaned in petroleum ether and dried. The wear scar diameter on the steel balls were measured using an optical microscope to an accuracy of ± 0.1 mm. The worn surface of the steel balls after friction test were observed through a JSM-5600LV scanning electron microscope (SEM, accelerating voltage 20 kv). Results and Discussion FTIR and TG analyses of TiO 2 nanoparticles surface-capped by WD-11 FTIR spectra of WD-11, bare TiO 2 and TiO 2 modified by WD-11 are shown in Fig. 1. The strong peaks of WD-11 at 2852 and 2926 cm -1 are assigned to the symmetric and asymmetric vibrations of CH 2 - and CH 3 groups, respectively; the sharp peak at 1468 cm -1 is due to the scissoring of CH 2 or the symmetric deformation of the CH 3 group; the peaks at 1180 cm -1 and 1087 cm -1 are due to the vibration absorption of Si-O-C and Si-O-Si, respectively. Besides, the strong peak of WD-11 at 818 cm -1 is assigned to the vibration of Si-C, and that at 725 cm -1 is assigned to the vibration of (CH 2 ) n (n is equal to or greater than 4). As to bare TiO 2, a very broad band emerges in a range of 700~500 cm -1, which is due to the vibration of Ti-O-Ti bonds in TiO 2 lattice 20. Different from bare TiO 2, TiO 2 nanoparticles surface-modified by WD-11 showed FTIR spectra varying with extract duration. Namely, non-extracted surface-capped TiO 2 sample showed obvious characteristic bands of WD-11 (Fig. 1a), confirming the successful surface-capping of TiO 2 nanoparticles by WD-11 containing long-chain aliphatic groups; the peaks at 1180 cm -1 disappeared, and a new absorption peak emerged at 995 cm -1, due to the vibration of Ti O Si 21. After surface-capped TiO 2 nanoparticles were washed and extracted for different durations, the characteristic bands of WD-11 became weaker (see Fig. 1b and Fig. 1c) 22. This implies that most physically bonded WD-11 molecules were removed by washing, but chemically bonded WD-11 mainly occupied on the surface of TiO 2 nanoparticles. Figure 2 shows the TG curves of surface-modified TiO 2 nanoparticles after being washed and extracted for different durations. As-obtained surface-capped TiO 2 nanoparticles showed the maximum weight loss (curve a in Fig. 2). The surface-capped TiO 2 nanoparticles obtained after three times of washing and 6 h of extraction had a lowered weight loss (curve b in Fig. 2). When the extraction time rose to 12 h, the weight loss of corresponding surface-capped TiO 2 nanoparticles was almost the same as that of the surface-capped TiO 2 nanoparticles obtained after three times of washing and 6 h of extraction (curve c Fig. 1 FTIR spectra of surface-modified TiO 2 nanoparticles after being washed and extracted for different durations: (a) no washing and extraction, (b) 3 washings and 6 h extraction and (c) 3 washings and 12 h extraction Fig. 2 TG curves of surface-modified TiO 2 nanoparticles after being washed and extracted for different durations: (a) without washing and extraction, (b) 3 washings and 6 h extraction, (c) 3 washings and 12 h extraction

564 INDIAN J. ENG. MATER. SCI., DECEMBER 2013 in Fig. 2). This indicates that the WD-11 physically adsorbed on the surface of TiO 2 nanoparticles had been mostly removed after three washing and 6 h extraction. The obtained modified TiO 2 nanoparticles were amorphous structure from the XRD spectrum, as shown in Fig. 3. The formation of amorphous structure may be partly attributed to large numbers of organic groups capped or adsorbed on its surface as well as the mixed silane oligomers, which also resulted in a high weight loss from above TGA measurements. The as-prepared amorphous TiO 2 nanoparticles were calcined at 200ºC, 400ºC and 600ºC, respectively. Anatate TiO 2 formed after the calcination at 400ºC, but a higher calcination temperature gives rise to the formation of the mixture of anatase TiO 2 and rutile TiO 2. Fig. 3 XRD spectra of surface-modified TiO 2 nanoparticles by different heat treatment TEM observation and measurement of UV-vis spectra of surface-modified TiO 2 nanoparticles Figure 4 shows the TEM micrographs of bare TiO 2 nanoparticles (Fig. 4a) and TiO 2 modified by WD-11 (Fig. 4b). Bare TiO 2 nanoparticles have a narrowly distributed size of 10~15 nm and tend to agglomerate severely when dispersed in organic solvents. Surface-modified TiO 2 nanoparticles have a relatively larger size (10~20 nm) but much better dispersibility in organic solvents than bare TiO 2 nanoparticles. The 18C long carbon chains formed a single or multi-layered molecular film on the surface of TiO 2 nanoparticles, which prevented the nanoparticles from contacting. Simultaneously the surface modifiers increased steric hindrance and reduced surface tension, so the WD-11 can greatly reduce the surface energy of TiO 2 nanoparticles and prevent them from aggregation. The dispersion stability of the colloidal solution of surface-modified TiO 2 nanoparticles in various organic solvents was evaluated based on UV-vis spectrometric analysis. Corresponding UV-vis transmittance spectra with a photograph (inset) of the solution (1%, w/v) of TiO 2 nanoparticles modified by WD-11 in alcohol, chloroform, paraffin liquid, dibutylphthalate, and toluene are shown in Fig. 5. It is seen that TiO 2 nanoparticles modified by WD-11 are well dispersed in paraffin liquid and dibutylphthalate but they tend to precipitate in alcohol and chloroform. Besides, surface-modified TiO 2 nanoparticles are dispersed in toluene but show slight turbidity. This means that the surface-modified TiO 2 nanoparticles possess better dispersibility in weakly polar organic solvents. Fig. 4 TEM images of (a) bare TiO 2 nanoparticles and (b) TiO 2 surface-modified by WD-11

PAN et al.: TIO 2 NANOPARTICLES SURFACE MODIFIED BY OCTADECYLTRIMETHOXYSILANE 565 Figure 6 shows the variation of wear scar diameter of the steel balls with the concentration of surface-modified TiO 2 nanoparticles in liquid paraffin, where the results under the lubrication of liquid paraffin alone as well as bare TiO 2 nanoparticles and pure WD-11 in liquid paraffin at the concentration of 0.5 % are also given for a comparison. The wear scar diameter of the steel balls under the lubrication of liquid paraffin alone is 0.684 mm, and it is reduced to 0.661 mm and 0.605 mm under the lubrication of bare TiO 2 and pure WD-11 in liquid paraffine. Surprisingly, the introduction of surface-capped TiO 2 nanoparticles in liquid paraffin led to significantly reduced wear scar diameter of the steel balls; and the lowest wear scar diameter of 0.407 mm was obtained under the lubrication of liquid paraffin containing 1% surface-capped TiO 2 nanoparticles. This demonstrates that surface-capped TiO 2 nanoparticles as an additive are able to effectively improve the antiwear ability of liquid paraffin; and it seems that there exists an optimal additive concentration at which the best antiwear performance is achieved. The reason may lie in that TiO 2 nanoparticles surface-capped by WD-11 is able to form a boundary lubricating and protective film containing long carbon chains on sliding steel surface. However, the wear scar diameter of the steel balls does not simply decline with increasing additive concentration, possibly because excessive amount of nanoparticles in paraffin liquid causes damage to the boundary lubricating and protective film. Figure 7 shows SEM images of the rubbing surface lubricated by paraffin oil and oil with WD-11/TiO 2 nanoparticles. It can be found that the worn surface lubricated by paraffin oil shown in Fig. 7a is evidently rough with many thick and deep furrows, but the worn surface lubricated by oil with WD-11/TiO 2 nanoparticles is comparably rather smoother and the furrows are rather shallower (Fig. 7b). This suggests that the oil with nanometer WD-11/TiO 2 possessed higher wear resistance than base oil. Mechanism of surface modification From the above analysis, a mechanism of formation of the organo-capped TiO 2 nanoparticles can be inferred as: as the addition drop-wise of the Ti(OC 4 H 9 ) 4, it hydrolyzed immediately and a TiO 2 or Ti(OH) 4 clear colloid was obtained. Fig. 5 UV-vis transmittance spectra along with photographs of the solutions (1%, w/v) of TiO 2 nanoparticles modified by WD-11 in (1) alcohol, (2) chloroform, (3) paraffin liquid, (4) dibutylphthalate and (5) toluene Fig. 6 Variations in wear scar diameter of steel ball with additive concentration in paraffin liquid (1450 r/min, 300 N, 30 min), the additive concentration of WD-11/TiO 2 in liquid paraffin were 0%, 0.1%, 0.3%, 0.5%, 1%, 2%, 4% and 6%, respectively and the concentrations of both bare TiO 2 and WD-11 in liquid paraffin were 0.5%.

566 INDIAN J. ENG. MATER. SCI., DECEMBER 2013 Fig. 7 SEM morphologies of worn steel surfaces lubricated with (a) liquid paraffin and (b) liquid paraffin with 1% WD-11/TiO 2 nanoparticles an organic layer. Figure 8 shows the structural model of WD-11-capped TiO 2 nanoparticles. The long aliphatic groups on the particles surface improve their hydrophobicity and allow them to form a stable dispersion in organic solvents. Fig. 8 Proposed structural model for WD-11-capped TiO 2 nanoparticles Octadecyltrimethoxysilane physico- or chemicoadsorbed on the particle surface, two reactions may have occurred at the same time: Both of these reactions would eliminate some of the water. Reaction (1) leads to the aggregation of the particles, while reaction (2), leads to formation of the Ti-OSiR(OCH 3 ) 2 bond, which would be more difficult to be attacked by water. If the Ti atom bonded to the silicate ion, it would not react as reaction (1) and the reaction went to completion. Finally, the inorganic nuclei should be capped with Conclusions TiO 2 nanoparticles encapsulated by octadecyltrimethoxysilane were prepared by in situ surface modification in dehydrated ethanol at room temperature. As-obtained surface-capped TiO 2 nanoparticles and those obtained after washing and extraction were characterized by means of FTIR, TEM, TGA, XRD and UV-vis, while their antiwear behavior as additive in liquid paraffin was evaluated. It has been found that surface-capped TiO 2 nanoparticles can be well dispersed in various organic solvents of weak polarity, showing potential for fabrication of inorganic/organic nanocomposites used in optical field. In the meantime, surface-capped TiO 2 nanoparticles are able to effectively improve the antiwear ability of liquid paraffin, showing promising application as a multifunctional lubricating oil additive. Acknowledgement This study was supported by the Natural Science Research Program of the Education Department of Henan Province, China (No. 2009B150004) and the Key Laboratory Foundation of Henan Province, China (No. 122300413205) References 1 Wang Y D, Zhang S & Wu X H, Nanotechnology, 15 (2004) 1162-1165. 2 Zhang Y P & Pan C X, J Mater Sci, 46 (2011) 2622-2626. 3 Mallakpour S & Barati A, Prog Org Coat, 71 (2011) 391-398.

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