1. IntroductionNanoparticles have many unique mechanical, magnetic, thermal, optical, catalytic properties, but its agglomeration due to high surface energy and surface activity hinders their application [1,2].
A method for solving this problem is to disperse nanoparticles in a base fluid, known as nanofluids, is studied for many years. The nanofluids is composed of two parts, including solvents and nanoparticles. The solvents of nanofluids are always water, oil, acetone, decene and ethylene glycol, and the nanoparticles used are usually metallic particles [3,4], metallic and nonmetallic oxides [5-7], carbon nanotube [8], etc. These conventional nanofluids improve the dispersion of nanoparticles to a certain extent, but the system is a kind of suspension and unstable, nanoparticles in the nanofluids may aggregate and settle down [9]. The factors influencing the stability and properties of nanofluids include the nanoparticle’s concentration, dispersant, viscosity of system [10], moreover, the variety, diameter [11,12], density of nanoparticle and ultrasonic vibration are not be ignored [13].
Recently, some researchers synthesize a new series of nanofluids which can flow at low temperature in absence of solvent (liquid) by surface modification. These solvent-free nanofluids involve SiO2 [14,15], TiO2 [16], CaCO3, C60 [17], ZnO [18], carbon naotube [19-21], etc. By the chemical reactions between active groups on the nanoparticles’ surface (always hydroxyl groups) and the organic modifier, an organic soft shell forms on the surface of nanoparticles, it can not only reduce the agglomeration of nanoparticles, but also impart new properties to them.
Actually, another method is to introduce the nanoparticle into block copolymer nanostructures. Prof. Ruckenstein and co-worker have been identified it [22,23].
In this paper, we select the organic reagent (CH3O)3 Si(CH2)3O(CH2CH2O)6–9CH3 to modify TiO2 nanoparticles, which is synthesized by sol-gel method. The silanol groups in the modifier can interact with hydroxyl groups on the surface of nanostructures, after a long reaction process, TiO2 nanoparticles are coated by a mass of organic molecular and a core-shell structure forms. The new system possesses much better dispersion and can flow at the room temperature.
2. Materials and Methods2.1. Raw MaterialsTetra-n-butyl titanate was purchased from TianJing KeMiou Chemical Company. Methanol (CH3OH, 99.5%), ethanol, HCl (36% - 38%), ammonia(NH4OH) and tetrahydrofuran were purchased as analytical grade reagents from Fuchen Chemical Ind., Ltd., and used without further purification. Deionized water was made in lab. (CH3O)3Si(CH2)3N+(CH3)(C10H21)2Cl− in methanol (40%) was from Gelest. C9H19-C6H4-(OCH2-CH2)20(CH2)3
K+ was from Sigma-aldrich.
2.2. Synthesis of TiO<sub>2</sub> NanoparticlesTiO2 nanoparticles were prepared by a sol-gel method through Tetrabutyl titanate hydrolysis. 17mL of Tetrabutyl titanate was mixed with 15mL of ethanol. The mixture was called as solution A. Solution B was prepared by mixing 15mL of ethanol, 2 mL of 5.5 mol/L hydrochloric acid solution, and1mL of deionized water. Then trickled solution B slowly to solution A with stiring constantly, and stop the experiment after the formation of gel. The gel was aged for 6 h at room temperature and carefully grinded after drying at 65˚C.
2.3. Synthesis of TiO<sub>2</sub> NanofluidsFor the TiO2 nanofluids, 0.5 g of TiO2 powder was dispersed in 10mL of ammonia (pH 10), the suspension was treated with ultrasonic for 30 min, then 2.5 g (CH3O)3 Si(CH2)3O(CH2CH2O)6–9CH3 was added. The mixture was placed in a sealed single-mouth flask and treated at 70˚C for 24 h. The final solution was extracted with toluene three times, the aqueous layer was collected and dried at 65˚C. The dried material was dispersed in 20mL of deionized water and extracted with toluene three times again. After collecting the aqueous layer, the solution was dried at 65˚C. Subsequently, the material was dispersed in 20 mL of the acetone, after centrifugation, the transparent sol was dried at 65˚C. The product is a yellow transparent liquid.
2.4. CharacterizationsThe structure of the TiO2 nanofluids was investigated by Fourier transform-infrared (FTIR) spectrometer analysis (WQF-310, Beijing Second Optical Instruments Factory) using KBr pellets. Transmission electron microscope (TEM) images were obtained on a Hitachi H-800 instrument at an accelerating voltage of 200 kV, placing a few drops of the dispersion on a copper grid, and evaporating them prior to observation. The thermogravimeric analysis (TGA) measurements were taken under N2 flow by using TA TGAQ50 instrument. Differential scanning calorimetry (DSC) traces were recorded collected on a TA Q1000 Instruments, heating rate of 10˚C/min, from −60˚C to 60˚C. Rheological properties were studied by using the rheometer of TA AR-G2 instrument, heating rate of 5˚C/min.
3. Results and DiscussionThe FTIR spectra of the TiO2 nanofluids are presented in Figure 1. The figure shows that they all have peak(s) at 450 cm−1 - 700 cm−1 which is the location of characteristic peaks of titania. The TiO2 nanofluids also have many new absorption peaks of organic groups compared with pure TiO2 nanoparticles. In theory, the reaction between TiO2 nanoparticles and (CH3O)3Si(CH2)3O(CH2CH2O)6–9 CH3 can yield Ti-O-Si, Si-O-Si bonds, from the spectra, their peaks are found at 944 cm-1 and 1110 cm-1 respectively [24]. In addition, the peak of stretching vibration of polyoxyethene is also observed at 1110 cm-1 overlapping with Si-O-Si. The strong peak at 3459 cm-1 is attributed to the presence of remaining hydroxyl groups on the TiO2 nanoparticles. The results prove that the modifier has been grafted on the surface of TiO2 nanoparticles.
The microstructure of the pure TiO2 nanoparticals and TiO2 nanofluids could be clearly observed from the TEM images (Figure 2). As shown in Figure 2, the pure TiO2 nanoparticals have serious phenomenon of agglomeration, its dispersion is significantly improved after modification. The modifier protects TiO2 nanoparticles from agglomeration and probably can improve its compatibility with organic materials.
Figure 3 is the DSC curve of the modifier (CH3O)3 Si(CH2)3O(CH2CH2O)6–9CH3 and the TiO2 nanofluids. In the heating process, both the modifier and TiO2 nanofluids show a second order transition at −50˚C, corresponding to the glass transition temperature (Tg). The first order transition of the modifier occurs at −0.4˚C, corresponding to the melting temperature (Tm). Differently, the TiO2 nanofluids has two first order transition at −27˚C and −3.6˚C, this may be the result of oligomeric siloxane of different molecular weight produced during the modification [22]. The two possess the same Tg (−50˚C), the